fix: make RestrictedPrior a distribution; extend test

[janfb]

What does this implement/fix? Explain your changes

Make RestrictedPrior a proper torch.Distribution with the required methods. The support is taken from the underlying base prior, or raised as not implemented.
This is fix the problem when using process_prior -- it will not be wrapped as a CustomPrior and it will also work with transforms, e.g., for MCMC-based posteriors.

Does this close any currently open issues?

Fixes #790

[michaeldeistler]

Amazing, thanks a ton!

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.31%. Comparing base (6dec812) to head (4226553).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
sbi/utils/restriction_estimator.py	72.72%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1257      +/-   ##
==========================================
- Coverage   86.05%   78.31%   -7.75%     
==========================================
  Files         118      119       +1     
  Lines        8672     8697      +25     
==========================================
- Hits         7463     6811     -652     
- Misses       1209     1886     +677     

Flag	Coverage Δ
unittests	78.31% <72.72%> (-7.75%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sbi/utils/restriction_estimator.py	76.80% <72.72%> (-8.86%)	⬇️

... and 30 files with indirect coverage changes