ENH: Moyal distribution

[fearghuskeeble]

I made a class for this distribution when I was working on my thesis. The use may be a little niche but it's at least popular enough to have a scipy class. I added documentation and testing for it so please add it if you would like.

Also included is a typo fix in the Student's T dist. documentation.

For reference the Moyal doc plot would look like this:

[twiecki]

Seems like travis is unhappy.

[fearghuskeeble]

Yes, I'd better check that one on docker because I don't get it locally.

[codecov]

Codecov Report

Merging #3870 into master will decrease coverage by 0.01%.
The diff coverage is 83.78%.

@@            Coverage Diff             @@
##           master    #3870      +/-   ##
==========================================
- Coverage   90.72%   90.71%   -0.02%     
==========================================
  Files         135      135              
  Lines       21264    21301      +37     
==========================================
+ Hits        19292    19323      +31     
- Misses       1972     1978       +6     

Impacted Files	Coverage Δ
pymc3/distributions/continuous.py	80.01% <76.92%> (-0.09%)	⬇️
pymc3/distributions/__init__.py	100.00% <100.00%> (ø)
pymc3/tests/test_distributions.py	98.16% <100.00%> (+<0.01%)	⬆️
pymc3/tests/test_distributions_random.py	98.52% <100.00%> (+0.01%)	⬆️

[fearghuskeeble]

I believe this is ready now

[twiecki]

Thanks @fearghuskeeble!