Add ImputationWarning class.

[rpgoldman]

The idea is that a programmer be able to ignore imputation warnings if they know that data is being imputed. It's easier to do this with a distinct class than with just UserWarning.

[codecov]

Codecov Report

Merging #3670 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3670      +/-   ##
==========================================
+ Coverage   89.77%   89.79%   +0.01%     
==========================================
  Files         134      134              
  Lines       20047    20050       +3     
==========================================
+ Hits        17998    18004       +6     
+ Misses       2049     2046       -3

Impacted Files	Coverage Δ
pymc3/model.py	87.75% <100%> (+0.01%)	⬆️
pymc3/exceptions.py	100% <100%> (ø)	⬆️
pymc3/distributions/distribution.py	95.32% <0%> (+0.29%)	⬆️
pymc3/step_methods/metropolis.py	87.81% <0%> (+0.31%)	⬆️
pymc3/step_methods/hmc/base_hmc.py	95.32% <0%> (+0.93%)	⬆️

[fonnesbeck]

Good idea. We should do something similar for the find_MAP warning, which essentially assumes the user is using the function incorrectly.

[lucianopaz]

Great idea! Thanks! If you want, you can do the find_MAP warning in another pr.