+Diag_mediator enhancements from ice shelf version

Added diag_mediator capabilities that had been developed and used in the SIS2 and MOM_ice_shelf versions of the diag mediator, in preparation for some of these separate versions to be combined into a single version. This includes adding an overload to the register_scalar_field() interface so that scalars can be registered with or without providing a null axis, retaining the existing interface while adding the one that is widely used with the ice shelf code. The previous routine register_scalar_field() is now register_scalar_field_CS(), but it has the same public interface.

This commit also adds the new public interface MOM_diag_send_complete() that calls the FMS routine that flushes the IO buffers. Although this call can be too expensive for production runs, it can be very useful for debugging.

Internally, register_diag_field() now checks for standard axes for 2-d fields, analogously to what was previously done for 3-d fields, thereby avoiding the allocation of a separate axis group for each 2-d diagnostic. This has been the case for ice-shelf code for some time, and it will save memory will giving identical results.

The testing for incompatible mask and array sizes was consolidated into a single call each in post_data_2d_low() and post_data_3d_low(), paving the way for adding the option of masking static fields.

All solutions are bitwise identical and diagnostic output (including metadata) is identical to what was previously output, but there are two new public interfaces.