Sampler utils

[astrolamb]

I removed the additional module I added from this PR that contained @paulthebaker's sampler utility functions.

I've added these utilities to sampler.py. This has allowed me to remove all of the various draw_from_x functions, and instead create a loop that create jump proposals if a given signal type is found in PTA model, based on the signal types created by enterprise.

I also added automatic pulsar grouping for better sampling of pulsar-dependent parameters.

Tests have been updated to reflect these changes.

TO DO:
JumpProposal should probably be cleaned up

[codecov-commenter]

Codecov Report

Merging #217 (537efe1) into master (541dd1f) will increase coverage by 2.85%.
The diff coverage is 66.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   37.23%   40.09%   +2.85%     
==========================================
  Files          20       20              
  Lines        3969     3614     -355     
==========================================
- Hits         1478     1449      -29     
+ Misses       2491     2165     -326     

Files	Coverage Δ
enterprise_extensions/hypermodel.py	50.52% <75.00%> (+2.22%)	⬆️
enterprise_extensions/sampler.py	61.65% <65.59%> (+23.64%)	⬆️

... and 1 file with indirect coverage changes

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