move BART to its own module

[aloctavodia]

Solves #4884. I do not agree with the characterization of BART as a "model architecture", I think is better to think about it as an stochastic process (or at least as something that approximated that). But GPs have the same description and they are also separated in its own module. Also this separation makes sense when in the near future we (I) add some utilities to work with BART.

[codecov]

Codecov Report

Merging #5058 (3994adc) into main (70f1975) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5058   +/-   ##
=======================================
  Coverage   78.33%   78.34%           
=======================================
  Files         129      130    +1     
  Lines       24427    24430    +3     
=======================================
+ Hits        19136    19140    +4     
+ Misses       5291     5290    -1     

Impacted Files	Coverage Δ
pymc/bart/bart.py	100.00% <ø> (ø)
pymc/bart/tree.py	100.00% <ø> (ø)
pymc/distributions/__init__.py	100.00% <ø> (ø)
pymc/step_methods/__init__.py	100.00% <ø> (ø)
pymc/__init__.py	96.07% <100.00%> (+0.07%)	⬆️
pymc/bart/__init__.py	100.00% <100.00%> (ø)
pymc/bart/pgbart.py	95.95% <100.00%> (ø)
pymc/sampling.py	87.43% <100.00%> (+0.01%)	⬆️
pymc/step_methods/hmc/nuts.py	95.00% <100.00%> (ø)
pymc/tests/test_bart.py	100.00% <100.00%> (ø)
... and 1 more

[michaelosthege]

Structure. I like it