Support regression for tree-based models

[iamDecode]

As gradient boosting uses regression trees internally, #23 already contained the majority of preparation for supporting regression in tree-based models. This PR adds two new classes PMMLTreeRegressor (extending DecisionTreeRegressor) and PMMLForestRegressor (extending RandomForestRegressor) for regression problems.

[codecov]

Codecov Report

Merging #25 (84f380b) into master (afb42d0) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master       #25    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           12        12            
  Lines          624       729   +105     
==========================================
+ Hits           624       729   +105     

Impacted Files	Coverage Δ
sklearn_pmml_model/ensemble/__init__.py	100.00% <100.00%> (ø)
sklearn_pmml_model/ensemble/forest.py	100.00% <100.00%> (ø)
sklearn_pmml_model/ensemble/gb.py	100.00% <100.00%> (ø)
sklearn_pmml_model/tree/__init__.py	100.00% <100.00%> (ø)
sklearn_pmml_model/tree/tree.py	100.00% <100.00%> (ø)

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