+FMS2 reads for ints & MOM_read_data_fms2 cleanup

  Added variants of MOM_read_data_0d, MOM_read_data_0d_int, MOM_read_data_1d,
MOM_read_data_1d_int, and MOM_read_data_2d_region that use the FMS2 interfaces
to read data.  Also altered prepare_to_read_var so that it does not open a file
(which had been an option before); one argument to prepare_to_read_var was no
longer needed and so it was removed.  Also added a public interface to
find_varname_in_file that does the same as prepare_to_read_var but works on the
other FMS2 file type without domain decomposition.  Several unused and now
redundant routines were removed from MOM_read_data_fms2.F90. Comments describing
a number of variables were also added.  All of these changes are confined to
config_src/infra/FMS2, and all answers are bitwise identical.