detect FORCE_DTSTEP based on MET_TAG (?)

[gmao-rreichle]

FORCE_DTSTEP depends on the forcing data set and could be determined from MET_TAG.

Rather than having FORCE_DTSTEP be a separate input in the "exeinp" file, it could be determined from a look-up table that pairs MET_TAG with the appropriate FORCE_DTSTEP value.

a) This could probably be done during ldas_setup. In this case, ldas_setup would still write FORCE_DTSTEP into LDAS.rc.

b) Alternatively, FORCE_DTSTEP could be removed entirely from LDAS.rc. In this case, GEOSldas would have to read and process the look-up table during the initialization of the MetForce GridComp. This option is probably more involved.

Option a) seems easier.

[gmao-rreichle]

In the meantime, some language was added to the documentation (see pull request #286 )
@gmao-jkolassa