Add core library to package

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Adds the majority of the clisops.core.subset content to the package.

Does this PR introduce a breaking change?

Some doc strings and logic have been modified to make better use of numpy.

Other information:

FYI @tlogan2000

The idea is to progressively move the core functionality into here, starting with the subsetting tools.

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