New treatment of ice shelf boundaries (#467)

Previously, ice-shelf Dirichlet boundary conditions only allowed u-velocity and v-velocity to be set to the same value (which is enforced by setting the value of parameters u_face_mask or v_face_mask to 3). This functionality is retained here, but now setting u_face_mask or v_face_mask to 5 enforces a Dirichlet boundary for u-velocity only along the respective cell face. Similarly, setting u_face_mask or v_face_mask to 6 enforces the Dirichlet boundary for v-velocity only along the respective cell face. This functionality is required for most ice-sheet modeling configurations, e.g. the idealized MISMIP+ configuration requires setting v-velocity only to 0 along its lateral boundaries, but with free-slip conditions enforced for u-velocity. Adding this capability required changes throughout the ice-shelf code.

Further changes were needed for how driving stress and Neummann conditions at computational boundaries are calculated in subroutine calc_shelf_driving_stress, and calls to subroutine apply_boundary_values were eliminated because they were not justified and caused errors. The new boundary treatment was tested by comparing simulated and analytical solutions for 1-D ice shelf flow with free-slip lateral boundary conditions and positive u_velocity enforced at the western boundary. This required the addition of a new parameter ADVECT_SHELF, which if false (as in the 1-D test case), turns off ice-shelf thickness evolution. By default, ADVECT_SHELF=True.

The new boundary treatment was also justified in 2-D by correctly simulating the expected MISMIP+ steady-state.