Fix strange ResourceWarning

[WilliamJamieson]

Closes #150

This PR should fix the seemingly "random" file handle left open errors. In a few cases the tests were not using a context manager nor calling close when opening a datamodel from a file.

While this normally calls the destructor __del__:

roman_datamodels/src/roman_datamodels/datamodels/_core.py

Lines 121 to 123 in c4b5d46

	def __del__(self):
	"""Ensure closure of resources when deleted."""
	self.close()

When the model in question goes out of scope, this is not ensured to happen immediately nor is in ensured to happen at the close of the interpreter, see the python docs. The seemingly "random" resource warning was detecting the cases where the models opened but not explicitly closed went out of scope, but the __del__ had not been called yet. This can occur for all sorts of arbitrary reasons during any given execution of the tests (e.g. system load, memory address space, etc).

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/romancal/XXX/ These only affect roman_datamodels tests.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ce423ee) 94.47% compared to head (6b0eb91) 94.47%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #231   +/-   ##
=======================================
  Coverage   94.47%   94.47%           
=======================================
  Files          23       23           
  Lines        1628     1628           
=======================================
  Hits         1538     1538           
  Misses         90       90           

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