Better handling of coordinates

[philbull]

This PR clarifies which coordinate systems are being assumed for the input variables (source position cosines crd_eq and time-dependent rotation matrices eq2tops), and fixes an ordering bug in a coordinate conversion for the non-pixel beam call.

[steven-murray]

Thanks @philbull, this is much needed. I've approved, but mentioned a couple of things to tidy up.

[codecov]

Codecov Report

Merging #10 (f04e19f) into main (be9df1b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #10   +/-   ##
=======================================
  Coverage   45.00%   45.00%           
=======================================
  Files           6        6           
  Lines         320      320           
  Branches       49       49           
=======================================
  Hits          144      144           
  Misses        162      162           
  Partials       14       14           

Impacted Files	Coverage Δ
src/vis_cpu/conversions.py	69.76% <ø> (ø)
src/vis_cpu/vis_cpu.py	80.51% <100.00%> (ø)

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