Split up gw likelihood

This MR splits up the bilby.gw.likelihood module into a subpackage as the file was getting very large.

I didn't do any reformatting of the classes/functions, but I did run black on the setup.py script.

Requires !1026 (merged) Closes #602 (closed)

changed milestone to %1.2.0

added Refactoring label

added 17 commits

• d3673598...c14eb63b - 16 commits from branch master
• 1ba82b4e - Merge remote-tracking branch 'origin/master' into split-up-gw-likelihood

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added 1 commit

• dd3c5e89 - updates from master

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added 3 commits

• dd3c5e89...d51d5194 - 2 commits from branch master
• 94d7483b - Merge remote-tracking branch 'origin/master' into split-up-gw-likelihood

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added 1 commit

• 4ddddcf3 - Flake fix

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approved this merge request

approved this merge request

merged

mentioned in commit 1b07e26f