+Add the optional argument old_name to get_param

  Added the new optional argument old_name to the 8 get_param() routines. This
new capability allows for an archaic parameter name to be specified and for
appropriate warnings encouraging the user to migrate to using the new name
while still setting the parameter as intended, or error messages in the case of
inconsistent setting via the archaic name and the correct name.  The logging
inside of the MOM_parameter_doc files only uses the correct parameter name.

  Also added the new optional argument set to the 8 read_param routines, to
indicate whether a parameter has been found and successfully set.  The new set
argument is now being used in read_param() calls in obsolete_int(),
obsolete_real(), obsolete_char() and obsolete_logical().  Obsolete_logical() in
particular was substantially simplified by the use of this new argument, and is
now only about half as long as it was.  The read_param() set argument is also
used in all of the get_param() routines when they are given an old_name
argument.

  The new old_name argument to get_param() is not yet being used in the version
of the MOM6 code that is being checked in, but it has been tested extensively
by adding or modifying get_param calls in a variant of the initialization code,
and it will be used in an updated version of github.com/NOAA-GFDL/pull/725
to gracefully handle the deprecation of 4 parameter names.

  All answers are bitwise identical, but there are new optional arguments to
two widely used interfaces.

src/diagnostics/MOM_obsolete_params.F90

@@ -168,29 +168,15 @@ subroutine obsolete_logical(param_file, varname, warning_val, hint)
   character(len=*), optional, intent(in) :: hint   !< A hint to the user about what to do.
   ! Local variables
   logical :: test_logic, fatal_err
+  logical :: var_is_set  ! True if this value was read by read_param.
   character(len=128) :: hint_msg
 
-  test_logic = .false. ; call read_param(param_file, varname, test_logic)
+  test_logic = .false. ; call read_param(param_file, varname, test_logic, set=var_is_set)
   fatal_err = .true.
-  if (present(warning_val)) fatal_err = (warning_val .neqv. .true.)
+  if (var_is_set .and. present(warning_val)) fatal_err = (warning_val .neqv. test_logic)
   hint_msg = " " ; if (present(hint)) hint_msg = hint
 
-  if (test_logic) then
-    if (fatal_err) then
-      call MOM_ERROR(FATAL, "MOM_obsolete_params: "//trim(varname)//   &
-           " is an obsolete run-time flag, and should not be used. "// &
-           trim(hint_msg))
-    else
-      call MOM_ERROR(WARNING, "MOM_obsolete_params: "//trim(varname)// &
-           " is an obsolete run-time flag. "//trim(hint_msg))
-    endif
-  endif
-
-  test_logic = .true. ; call read_param(param_file, varname, test_logic)
-  fatal_err = .true.
-  if (present(warning_val)) fatal_err = (warning_val .neqv. .false.)
-
-  if (.not.test_logic) then
+  if (var_is_set) then
     if (fatal_err) then
       call MOM_ERROR(FATAL, "MOM_obsolete_params: "//trim(varname)//   &
            " is an obsolete run-time flag, and should not be used. "// &
@@ -211,12 +197,13 @@ subroutine obsolete_char(param_file, varname, warning_val, hint)
   character(len=*), optional, intent(in) :: hint  !< A hint to the user about what to do.
   ! Local variables
   character(len=200) :: test_string, hint_msg
+  logical :: var_is_set  ! True if this value was read by read_param.
   logical :: only_warn
 
-  test_string = ''; call read_param(param_file, varname, test_string)
+  test_string = ''; call read_param(param_file, varname, test_string, set=var_is_set)
   hint_msg = " " ; if (present(hint)) hint_msg = hint
 
-  if (len_trim(test_string) > 0) then
+  if (var_is_set) then
     only_warn = .false.
     if (present(warning_val)) then ! Check if test_string and warning_val are the same.
       if (len_trim(warning_val) == len_trim(test_string)) then
@@ -246,15 +233,16 @@ subroutine obsolete_real(param_file, varname, warning_val, hint, only_warn)
 
   ! Local variables
   real :: test_val, warn_val
+  logical :: var_is_set  ! True if this value was read by read_param.
   logical :: issue_warning
   character(len=128) :: hint_msg
 
-  test_val = -9e35; call read_param(param_file, varname, test_val)
+  test_val = -9e35; call read_param(param_file, varname, test_val, set=var_is_set)
   warn_val = -9e35; if (present(warning_val)) warn_val = warning_val
   hint_msg = " " ; if (present(hint)) hint_msg = hint
   issue_warning = .false. ; if (present(only_warn)) issue_warning = only_warn
 
-  if (test_val /= -9e35) then
+  if (var_is_set) then
     if ((test_val == warn_val) .or. issue_warning) then
       call MOM_ERROR(WARNING, "MOM_obsolete_params: "//trim(varname)// &
          " is an obsolete run-time flag. "//trim(hint_msg))
@@ -273,14 +261,15 @@ subroutine obsolete_int(param_file, varname, warning_val, hint)
   integer,     optional, intent(in) :: warning_val !< An allowed value that causes a warning instead of an error.
   character(len=*), optional, intent(in) :: hint   !< A hint to the user about what to do.
   ! Local variables
+  logical :: var_is_set  ! True if this value was read by read_param.
   integer :: test_val, warn_val
   character(len=128) :: hint_msg
 
-  test_val = -123456788; call read_param(param_file, varname, test_val)
+  test_val = -123456788; call read_param(param_file, varname, test_val, set=var_is_set)
   warn_val = -123456788; if (present(warning_val)) warn_val = warning_val
   hint_msg = " " ; if (present(hint)) hint_msg = hint
 
-  if (test_val /= -123456788) then
+  if (var_is_set) then
     if (test_val == warn_val) then
       call MOM_ERROR(WARNING, "MOM_obsolete_params: "//trim(varname)// &
          " is an obsolete run-time flag. "//trim(hint_msg))