Fix StanMetaRegression estimator

[JulioAPeraza]

Closes #67.

Changes proposed in this pull request:

• Update the StanMetaRegression code to work with the new PyStan's API (PyStan 3).
• Remove warning.
• Enable test_stan_estimators.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.85%. Comparing base (1402199) to head (e56e294).
Report is 7 commits behind head on master.

Files	Patch %	Lines
pymare/estimators/estimators.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   87.79%   90.85%   +3.06%     
==========================================
  Files          13       13              
  Lines         893      897       +4     
==========================================
+ Hits          784      815      +31     
+ Misses        109       82      -27     

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[JulioAPeraza]

Tests on Python 3.6 are failing because PyStan 3 only works for Python 3.7+. @tsalo, do you think we should remove the test on 3.6 completely, or there is a way to just skip 3.6 for test_stan_estimators?

The other tests on ubuntu-latest (20.04.4) failed with an error that was already reported here: https://discourse.mc-stan.org/t/schools-example-not-compiling-pystan-3-4-on-ubuntu-20-04-4/27812.

[tsalo]

I'm thinking that we should (1) mark the pystan tests with a decorator and try to skip them with 3.6, (2) ensure that pystan is an optional dependency, (3) include in our documentation that users should not try using pystan with PyMARE and Python 3.6, and (4) add some type of warning based on the import and Python version within the StanMetaRegression estimator at initialization.

[JulioAPeraza]

Thanks! I have added the decorators and included pystan as an optional dependency and the tests are now passing. I still need to find what's happening to the test on ubuntu-latest (20.04.4).

[JulioAPeraza]

Now all tests are passing after switching to ubuntu 20.04 for Python 3.6.
@tsalo Is there anything else you would like to see in this PR?

[jdkent]

LGTM, thanks @JulioAPeraza!