Support basin constraint in mappings

[matthew-mizielinski]

At present the list of constraints supported in a mapping is limited and does not include "basin", see

CDDS/mip_convert/mip_convert/common.py

Lines 29 to 30 in 6a4a721

	SUPPORTED_NETCDF_CONSTRAINTS = ['variable_name', 'cell_methods', 'depth']
	SUPPORTED_CONSTRAINTS = SUPPORTED_PP_CONSTRAINTS + SUPPORTED_NETCDF_CONSTRAINTS

We need to identify whether any additional code is needed to support basin constraints in MIP convert in order to support mappings such as UKNCSP/CDDS-CMIP7-mappings#1443

[mo-laurenboon]

In addition to this, there is also a constraint "olevel" that is currently unsupported. This appears to be a similar issue to the one listed above. Should this be completed in conjunction with this or as a separate issue?

Example variable: UKNCSP/CDDS-CMIP7-mappings#1567