Enable Meta Pixels to be generated from just top or bottom pixels

[DanRyanIrish]

This PR enables users to generate their meta pixels for use in calculating STIX visibilities by only using top or bottom big pixels. This is needed in cases where the flare occurred at the edge of the STIX FOV, resulting in one row of pixels not being fully illuminated.

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.47%. Comparing base (5dbfc0b) to head (7794e76).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
stixpy/calibration/visibility.py	95.65%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   73.56%   74.47%   +0.90%     
==========================================
  Files          32       32              
  Lines        2016     2029      +13     
==========================================
+ Hits         1483     1511      +28     
+ Misses        533      518      -15     

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[DanRyanIrish]

ping @samaloney for a review.

[samaloney]

@DanRyanIrish made some small changes I'll merge if you are happy?