small mwax metafits updates

[PyxieLouStar]

Update how instrument and Nants_telescope are pulled from the metafits file.

Description

Instead of using the TELESCOP field in the metafits file to set instrument, use the INSTRUME field instead. Instead of using the INSTRUME field to determine Nants_telescope, set it as the number of antennas present in the metafits antenna table. The MWAX test metafits file has been updated with the appropriate INSTRUME field.

Motivation and Context

Closes #1300

The definition of the INSTRUME field has been updated with the implementation of the MWAX correlator. For the Legacy correlator, this field indicated the number of antennas present in the array. With MWAX, this assumption is incorrect and so a different method for computing Nants_telescope is required.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• 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:

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

Bug fix checklist:

• My fix includes a new test that breaks as a result of the bug (if possible).
• All new and existing tests pass.
• I have updated the CHANGELOG.

[bhazelton]

Looks good, thanks!

[codecov]

Codecov Report

Merging #1299 (cb6c400) into main (9481753) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1299   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          36       36           
  Lines       20062    20062           
=======================================
  Hits        20046    20046           
  Misses         16       16           

Impacted Files	Coverage Δ
pyuvdata/uvdata/mwa_corr_fits.py	100.00% <100.00%> (ø)

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[bhazelton]

Looks good!