fix: warn about using use_pixel_beams

[steven-murray]

This sets the default value of use_pixel_beams to False, because it's a bad idea.

Since it changes the API (in the sense of a default change), but I didn't want to make it a major-version thing, I emit a warning when the default value is used.

[codecov]

Codecov Report

Merging #222 (7c71f29) into main (9ba2f10) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   96.54%   96.55%           
=======================================
  Files          24       24           
  Lines        2809     2813    +4     
=======================================
+ Hits         2712     2716    +4     
  Misses         97       97           

Impacted Files	Coverage Δ
hera_sim/visibilities/vis_cpu.py	93.57% <100.00%> (+0.18%)	⬆️

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[r-pascua]

This looks good to me. I agree that this it feels silly to make a new major version release just for changing a default parameter, and that raising a warning is a nice middle ground. Thanks!