Collections Update

[rbeyer]

Description

Created a new MutableMappingSequence ABC for the pvl library to use, which the pvl.OrderedMultiDict already is, and made other collections-related updates.

Related Issue

Might close #62.

How Has This Been Tested?

• make lint
• make docs
• make test

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.