Split BART into its own submodule #4884
Description
Description of your problem
BART is currently contained in the distributions/ submodule which doenst quite made sense.
Currently BART is part of distributions/ in three modules
https://github.com/pymc-devs/pymc3/blob/main/pymc3/distributions/bart.py
https://github.com/pymc-devs/pymc3/blob/main/pymc3/distributions/tree.py
plus the PG sampler inside step method
Refactor request is to make BART its own submodule like GP and ODE are since BART is more of a "model architecture" than a distribution