Add base classes for OpenMC and Nek objects

[nuclearkevin]

Closes #1036

[moosebuild]

Job Documentation, step Sync to remote on d87cb59 wanted to post the following:

View the site here

This comment will be updated on new commits.

[moosebuild]

Job Coverage, step Generate coverage on d87cb59 wanted to post the following:

Coverage

	928d9f	#1038 d87cb5
	Total	Total	+/-	New
Rate	92.87%	92.88%	+0.01%	100.00%
Hits	8109	8110	+1	104
Misses	623	622	-1	0

Diff coverage report

Full coverage report

This comment will be updated on new commits.

[nuclearkevin]

@aprilnovak I've updated this PR to account for the changes made in #923. Once tests pass it should be ready for review.

[moosebuild]

Job Precheck, step Clang format on 1dfc85f wanted to post the following:

Your code requires style changes.

A patch was auto generated and copied here
You can directly apply the patch by running, in the top level of your repository:

curl -s https://mooseframework.inl.gov/cardinal/docs/PRs/1038/clang_format/style.patch | git apply -v

Alternatively, with your repository up to date and in the top level of your repository:

git clang-format f50dc82cc0e8ba80a4ef7b712de146c1f788e59b