Bug in `calc_fgdms` when doing <yearly processing

[mo-gill]

I'm guessing this is low priority as it feels like a rare edge case, so this is mostly just to record some findings if needed.

I ran a request with < 1year start and end date to save some time when iterating development and fgdms_tavg-u-hxy-sea was failing with this error.

*** Starting conversions ***
Loading data for "CMIP7_ocnBgchem.json: fgdms_tavg-u-hxy-sea"
Unable to produce MIP requested variable "fgdms_tavg-u-hxy-sea" for "CMIP7_ocnBgchem": Field and Land Fraction dimensions differ
Unable to produce MIP requested variable "fgdms_tavg-u-hxy-sea" for "CMIP7_ocnBgchem": Field and Land Fraction dimensions differ
Field and Land Fraction dimensions differ
Traceback (most recent call last):
  File "(redacted)/CDDS/mip_convert/mip_convert/request.py", line 124, in convert
    produce_mip_requested_variable(variable_name, stream_id, substream, mip_table, user_config,
  File "(redacted)/729_collect_ukcm2_ll_ancils/CDDS/mip_convert/mip_convert/requested_variables.py", line 132, in produce_mip_requested_variable
    time_slice.process()
  File "(redacted)29_collect_ukcm2_ll_ancils/CDDS/mip_convert/mip_convert/new_variable.py", line 361, in process
    self._apply_expression()
  File "(redacted)t_ukcm2_ll_ancils/CDDS/mip_convert/mip_convert/new_variable.py", line 552, in _apply_expression
    self.cube = plugin.evaluate_expression(expression, self.input_variables)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/(redacted)ncils/CDDS/mip_convert/mip_convert/plugins/ukcm2/ukcm2_plugin.py", line 40, in evaluate_expression
    return eval(expression)
           ^^^^^^^^^^^^^^^^
  File "<string>", line 1, in <module>
  File "(redacted)ds/729_collect_ukcm2_ll_ancils/CDDS/mip_convert/mip_convert/plugins/base/data/processors.py", line 1919, in calc_fgdms
    raise RuntimeError('Field and Land Fraction dimensions differ')
RuntimeError: Field and Land Fraction dimensions differ`

When run >year start and end date it produces fine.

I think it's due to the indexing going on here:

CDDS/mip_convert/mip_convert/plugins/base/data/processors.py

Lines 1916 to 1918 in 6a4a721

	# Check that the input cube and land fraction dimensions match
	if cube[0, ...].shape != land_fraction.shape:
	raise RuntimeError('Field and Land Fraction dimensions differ')

Here are the input cubes (before that indexing line) if <yearly request is used:
5month cube shapes = (144, 192), land_fraction shape = (144, 192)
And if 5 year request is used:
5year cube shapes = (12, 144, 192), land_fraction shape = (144, 192)

So i'm guessing to fix an adjustment would need to be made to the calc_fgdms processor.