Akp mt updates

[paulsonak]

Small updates for multitask model evaluation. See #382 for more info.

1. Update compare_models to check for multiple dataset_keys instead of multiple response_columns
2. Update MinimalDataset to calculate weights for training data predictions / metrics calculation.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
atomsci/ddm/pipeline/model_datasets.py	33.33%	2 Missing ⚠️
atomsci/ddm/pipeline/compare_models.py	0.00%	1 Missing ⚠️

@@            Coverage Diff             @@
##            1.7.0     #383      +/-   ##
==========================================
+ Coverage   20.29%   22.84%   +2.54%     
==========================================
  Files          47       47              
  Lines       12883    12885       +2     
==========================================
+ Hits         2615     2943     +328     
+ Misses      10268     9942     -326     

Files with missing lines	Coverage Δ
atomsci/ddm/pipeline/compare_models.py	5.64% <0.00%> (ø)
atomsci/ddm/pipeline/model_datasets.py	47.69% <33.33%> (-0.20%)	⬇️

... and 5 files with indirect coverage changes