rename O<x> to Omega_<x>

[nstarman]

Follows the heuristic of intuitive naming.

PR Checklist

• Give a detailed description of the PR above.
• Document changes in the CHANGES.rst file. See existing changelog for examples.
• Add tests, if applicable, to ensure code coverage never decreases.
• Make sure the docs are up to date, if applicable, particularly the docstrings and RST files in docs folder.
• Ensure linear history by rebasing, when requested by the maintainer.

[codecov-commenter]

Codecov Report

Merging #37 (53e93fe) into main (9666120) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   76.53%   76.53%           
=======================================
  Files          13       13           
  Lines         277      277           
=======================================
  Hits          212      212           
  Misses         65       65           

Impacted Files	Coverage Δ
src/cosmology/api/_background.py	68.42% <100.00%> (ø)
src/cosmology/api/_components.py	86.79% <100.00%> (ø)
src/cosmology/api/_standard.py	89.47% <100.00%> (ø)

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[nstarman]

I think this PR is ready.