Add datamodel maker

[WilliamJamieson]

When developing tests in romancal it was a little annoying to construct the datamodel you needed, as the maker_utils/testing.factories all construct the stnode objects, not the actual datamodels themselves.

This PR provides a convenience function (maker_utils.mk_datamodel) to do this for you. For any datamodel type (in the MODEL_REGISTRY) it automatically grabs the correct maker_util and then uses the it to construct data which it then feeds into the datamodel class to construct a full fledged datamodel of the desired type.

For example if one wants a RampModel the following will construct a RampModel filled with dummy data:

from roman_datamodels.datamodels import RampModel
from roman_datamodels.maker_utils import mk_datamodel

model = mk_datamodel(RampModel)

Note that this is based on #193 and so it will remain a draft for now.

Checklist

• Added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/Roman-devdeps/310/

[WilliamJamieson]

This is the actual magic of this whole PR. This will automatically pick up any new Datamodels added to the MODEL_REGISTRY with no additional developer input outside of providing a maker_utils function for the stnode structure, which the other tests require them to do.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10 🎉

Comparison is base (4a8f808) 97.84% compared to head (26c45e5) 97.94%.

❗ Current head 26c45e5 differs from pull request most recent head e2d3f86. Consider uploading reports for the commit e2d3f86 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
+ Coverage   97.84%   97.94%   +0.10%     
==========================================
  Files           8        9       +1     
  Lines        1204     1263      +59     
==========================================
+ Hits         1178     1237      +59     
  Misses         26       26              

Impacted Files	Coverage Δ
tests/test_maker_utils.py	97.82% <100.00%> (+0.26%)	⬆️
tests/test_models.py	98.32% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[PaulHuwe]

LGTM - like this addition