Factor out torch autograd checks into separate tests and skip on PR workflow runs

[matt-graham]

Resolves #294

Factors out the torch autograd checks in precompute spherical and Wigner transform test modules in to separate tests, with some additional refactoring to reduce code duplication at the same time. These separate tests are also given a 'slow' mark and this used to skip these tests in GitHub Actions tests workflow when triggered by commits on a pull-request, while still running them for other triggers such as pushes to main and for scheduled tests. This should hopefully give us benefit of much quicker tests on PRs while still giving some visibility on whether these tests are passing regularly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.55%. Comparing base (922dae6) to head (a3f7cf5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #304      +/-   ##
==========================================
- Coverage   96.60%   96.55%   -0.06%     
==========================================
  Files          32       32              
  Lines        3450     3450              
==========================================
- Hits         3333     3331       -2     
- Misses        117      119       +2     

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[jasonmcewen]

Haven't reviewed super thoroughly but LGTM