Rename num_output_group -> num_class, random_forest_flag -> average_tree_output (part of #229)

[hcho3]

Extracted from #229 for easier reviewing.

[codecov-io]

Codecov Report

Merging #233 (d50fdf6) into mainline (a4fa0a2) will decrease coverage by 0.05%.
The diff coverage is 95.38%.

@@              Coverage Diff               @@
##             mainline     #233      +/-   ##
==============================================
- Coverage       83.79%   83.73%   -0.06%     
  Complexity         44       44              
==============================================
  Files              93       93              
  Lines            6780     6855      +75     
  Branches           42       42              
==============================================
+ Hits             5681     5740      +59     
- Misses           1075     1091      +16     
  Partials           24       24              

Impacted Files	Coverage Δ	Complexity Δ
include/treelite/frontend.h	90.00% <ø> (ø)	0.00 <0.00> (ø)
include/treelite/tree.h	93.02% <ø> (ø)	0.00 <0.00> (ø)
python/treelite/sklearn/gbm_classifier.py	93.75% <ø> (ø)	0.00 <0.00> (ø)
python/treelite/sklearn/gbm_multi_classifier.py	94.11% <ø> (ø)	0.00 <0.00> (ø)
python/treelite/sklearn/gbm_regressor.py	93.75% <ø> (ø)	0.00 <0.00> (ø)
python/treelite/sklearn/rf_classifier.py	100.00% <ø> (ø)	0.00 <0.00> (ø)
python/treelite/sklearn/rf_multi_classifier.py	100.00% <ø> (ø)	0.00 <0.00> (ø)
python/treelite/sklearn/rf_regressor.py	100.00% <ø> (ø)	0.00 <0.00> (ø)
...main/java/ml/dmlc/treelite4j/java/TreeliteJNI.java	45.45% <ø> (ø)	2.00 <0.00> (ø)
src/compiler/ast/builder.h	100.00% <ø> (ø)	0.00 <0.00> (ø)
... and 22 more

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