Add allowMissing kwarg to loadOperator

[onufer]

Description

Adds kwarg to loadOperator to support more easily loading databases that have parameters not defined within the current ARMI version. This is helpful for backward compatibility when post-processing a database that had a param that was removed from ARMI .

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Checklist

• Tests have been added/updated to verify that the new or changed code works.
• Docstrings were included and updated as necessary
• The code is understandable and maintainable to people beyond the author
• There is no commented out code in this PR.

If user exposed functionality was added/changed:

• Documentation added/updated in the doc folder.
• New or updated dependencies have been added to setup.py.

[john-science]

I'll wait for the unit tests to finish running before approving.

But this looks like a very reasonable change.

It might be nice to have a unit test that flexes this for allowMissing" = TrueandFalse. There is already one unit test that uses loadOperator()`:

armi/armi/tests/test_lwrInputs.py

Lines 91 to 95 in b1c18bd

	o = armi_init(fName=TEST_INPUT_TITLE + ".yaml")
	locsInput, locsDB = {}, {}
	loadLocs(o, locsInput)
	o.operate()
	o2 = db.loadOperator(TEST_INPUT_TITLE + ".h5", 0, 0)

[onufer]

@john-science, Yes, the main reason i didn't add a test is that to trully test the feature I would need a database that had params that don't exist in the framework. I dont really want to commit another database to the framework, and dont want to write a test that doesn't actually test the feature. Open to feedback though.

[john-science]

@onufer Is it true that we need a whole need database to test this?

Couldn't we just create a new test YAML file that lists the parameters we need, to exclude on of the parameters in the default test database?

Sorry, maybe I'm missing something.

[onufer]

@john-science, you can test the method signature but not the feature, since the w/o a new db it wouldn't fail either way.