Version 3 -- Endgame

[kartographer]

"I am inevitable" -- V3 of pyuvdata

Description

Implements the final set of changes for pyuvdata version 3.0, primarily:

• Dropping support for "current array shapes" (the old spw-axis -- which was forced to always be 1 -- has been removed from attributes that have it)
• Requiring flexible spectral windows be used in UVData and UVCal` objects (the latter only when a frequency axis is present, i.e., non-wideband solutions)
• Dropping support for the "old" phasing parameters (which were deprecated when phase_center_catalog became a standard parameter).

Relatedly, the use_future_array_shapes method has been deprecated, and is planned to be removed in v3.2.

Note that while nearly all the required work on this branch is complete, I haven't updated the CHANGELOG yet, mostly in anticipation of (perhaps unlikely) significant changes that may be requested. Also, this PR will follow #1422, and will be moved to "ready" once that PR is successfully merged into prep_v3.0.

Motivation and Context

This PR is meant to be the final PR to close out V3 updates, and as such should be the final one merged into prep_v3.0 before that branch is ready or merge into main. And I for one am ready to overthrow our vestigal spw-axis overlords.

Types of changes

• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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

Breaking change checklist:

• [x] I have updated the docstrings associated with my change using the numpy docstring format.
• [x] 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.
• [x] All new and existing tests pass.
• [ ] I have updated the CHANGELOG.