refactor: use matvis insteaf of vis_cpu

[steven-murray]

Refactors hera_sim to use the matvis name instead of vis_cpu.

[piyanatk]

Thanks Steven. This looks good. Just need some tidy up by replaceing occurrences of vis_cpu with matvis in the comments/docstrings, I think.

[codecov]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (9b0f8b8) 92.89% compared to head (9c66df9) 92.89%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
hera_sim/visibilities/matvis.py	84.78%	18 Missing and 10 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #286   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files          24       24           
  Lines        3222     3222           
  Branches      705      705           
=======================================
  Hits         2993     2993           
  Misses        126      126           
  Partials      103      103           

Flag	Coverage Δ
unittests	92.86% <84.86%> (ø)

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