Prevent array shape mismatch due to NumPy squeeze in mhd_waves_.py

[TrestanSimon]

Description

This PR adds an if statement to prevent array shape mismatch due to np.squeeze in mhd_waves_.py.

np.squeeze will convert column vectors to row vectors, for example, which causes problems where the array shape is assumed to be preserved.

[github-actions]

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[namurphy]

Thank you for doing this, and apologies for the delay in merging it!

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 94.36%. Comparing base (71663fb) to head (e529624).
Report is 41 commits behind head on main.

Files	Patch %	Lines
src/plasmapy/dispersion/analytical/mhd_waves_.py	33.33%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2715      +/-   ##
==========================================
- Coverage   95.14%   94.36%   -0.78%     
==========================================
  Files         107      107              
  Lines        9411     9608     +197     
  Branches     2168     2225      +57     
==========================================
+ Hits         8954     9067     +113     
- Misses        276      347      +71     
- Partials      181      194      +13     

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