MultiDict implementation

[rbeyer]

Description

• Implemented PVLMultiDict which is based on the 3rd Party multidict.MultiDict object as an option to use instead of the default OrderedMultiDict. The new PVLMultiDict is better aligned with the Python 3 way that Mapping objects behave.
• Enhanced the existing OrderedMultiDict with some functionality that extends its behavior closer to the Python 3 ideal, and inserted warnings about how the retained non-Python-3 behaviors might be removed at the next major patch.
• Implemented pvl.new that can be included for those that wish to try out what getting the new PVLMultiDict returned from the loaders might be like by just changing an import statement.

Motivation and Context

Would close #52.

How Has This Been Tested?

• make lint
• make docs
• make test-all

Types of changes

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Licensing:

This project is released under the LICENSE.

• I claim copyrights on my contributions in this pull request, and I provide those contributions via this pull request under the same license terms that the pvl project uses.