Updates to support MIR v6 file format

[kartographer]

Some minor updates to support the newest file format for MIR.

Description

A total of four fields in the spectral window header object (MirSpData) have been updated to their new V6 names/types. Additionally, SMA targets now have their coordinates correcty labeled as being in the "ICRS" frame rather than FK5.

Motivation and Context

The fields within the MirSpData class have been updated to reflect their new allocations as of the most recent update to the file format. Fields being updated were previously marked as "spare", and thus the updates should have no impact on backward compatibility of datasets (rather they simply allow various pieces of software using MirParser to correctly access these fields from new data sets). Note that these changes are very low level from the perspective of UVData -- I don't expect the average user to ever encounter these changes.

The FK5/ICRS mixup was noticed only recently as part of testing related to the updated phasing memo, where thanks to the improved astrometry of both pyuvdata and the SMA online system, we were able to detect an apparent ~15 mas discrepancy between the two was traced back to this mislabeling.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

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

New feature checklist:

• 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).
• I have added tests to cover my new feature.
• All new and existing tests pass.
• I have updated the CHANGELOG.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (25c2ab8) to head (7cb8ec3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1416   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          37       37           
  Lines       20807    20807           
=======================================
  Hits        20791    20791           
  Misses         16       16           

Files	Coverage Δ
pyuvdata/uvdata/mir.py	100.00% <ø> (ø)
pyuvdata/uvdata/mir_meta_data.py	100.00% <ø> (ø)

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[e-koch]

@kartographer -- One minor point regarding the MIR meta data attributes in the test. Otherwise, looks good.