Diag manager: Allow scalar static variables

[uramirez8707]

Is your feature request related to a problem? Please describe.
The current register_static_field interface does not allow for scalar static variables.

FMS/diag_manager/diag_manager.F90

Lines 487 to 492 in 42f8506

	INTEGER FUNCTION register_static_field(module_name, field_name, axes, long_name, units,&
	& missing_value, range, mask_variant, standard_name, DYNAMIC, do_not_log, interp_method,&
	& tile_count, area, volume, realm)
	CHARACTER(len=*), INTENT(in) :: module_name !< Name of the module, the field is on
	CHARACTER(len=*), INTENT(in) :: field_name !< Name of the field
	INTEGER, DIMENSION(:), INTENT(in) :: axes !< Axes_id of the field

This will force users to have a dummy axis of size 1 which is terrible practice.
https://github.com/uramirez8707/FMS/blob/008ebd513cd170c223d6d50be9ff36bee156e3e3/test_fms/diag_manager/test_output_every_freq.F90#L50
https://github.com/uramirez8707/FMS/blob/008ebd513cd170c223d6d50be9ff36bee156e3e3/test_fms/diag_manager/test_output_every_freq.F90#L53

Describe the solution you'd like
The register_static_field should be expanded to allow for scalar static variables.

Describe alternatives you've considered
Forcing people to pass in a dummy axis of size 1

Additional context
N/A