Add metadata & support for flexible spws on UVFlag

[bhazelton]

Description

Add new metadata to UVFlag objects and support for flexible spectral windows.

Motivation and Context

closes #916 closes #1185

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation change (documentation changes only)
• [ ] Version change
• [ ] Build or continuous integration change

Checklist:

• [x] I have read the contribution guide.
• [x] My code follows the code style of this project.

New feature checklist:

• [x] I have added or updated the docstrings associated with my feature using the numpy docstring format.
• [ ] I have updated the tutorial to highlight my new feature (if appropriate).
• [x] I have added tests to cover my new feature.
• [ ] All new and existing tests pass.
• [ ] I have updated the CHANGELOG.

Breaking change checklist:

• [x] I have updated the docstrings associated with my change using the numpy docstring format.
• [ ] I have updated the tutorial to reflect my changes (if appropriate).
• [x] My change includes backwards compatibility and deprecation warnings (if possible).
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests pass.
• [ ] I have updated the CHANGELOG.