--Expose Attributes save functionality to python; cleanup/improve attribute registration.

[jturner65]

Motivation and Context

This PR exposes the attributes save-to-disk functionality via python; It expands the possible saving capabilities so that both directories and filenames can be independently specified. It also addresses some longstanding inefficiencies and potential issues with attribute registration

How Has This Been Tested

All C++ and python tests pass locally

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have completed my CLA (see CONTRIBUTING)
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.