Fix PGI runtime issue with class(*)

- Some tests such as global_ALE_z crash under PGI (ncrc4.pgi20 or
ncrc5.pgi227) with
FATAL from PE    27: unsupported attribute type: get_variable_attribute_0d: file:INPUT/tideamp.nc- variable:GRID_X_T attribute: axis
- PGI in general has issues with class(*) construct and in this case
cannot recognize the axis argument to be a string.
- This mod helps PGI recognize that the argument is a string.

config_src/infra/FMS2/MOM_io_infra.F90

@@ -1524,9 +1524,9 @@ subroutine categorize_axes(fileObj, filename, ndims, dim_names, is_x, is_y, is_t
       if (variable_exists(fileobj, trim(dim_names(i)))) then
         cartesian = ""
         if (variable_att_exists(fileobj, trim(dim_names(i)), "cartesian_axis")) then
-          call get_variable_attribute(fileobj, trim(dim_names(i)), "cartesian_axis", cartesian)
+          call get_variable_attribute(fileobj, trim(dim_names(i)), "cartesian_axis", cartesian(1:1))
         elseif (variable_att_exists(fileobj, trim(dim_names(i)), "axis")) then
-          call get_variable_attribute(fileobj, trim(dim_names(i)), "axis", cartesian)
+          call get_variable_attribute(fileobj, trim(dim_names(i)), "axis", cartesian(1:1))
         endif
         cartesian = adjustl(cartesian)
         if ((index(cartesian, "X") == 1) .or. (index(cartesian, "x") == 1)) is_x(i) = .true.