Handle unseen labels in multiclass classification for PDP

[rjagtani]

Fixes

1. Handles unseen labels in multiclass classification for PDP
2. Added option to not update storage in PDP
3. Added is_classification flag in PDP

[rjagtani]

Also two new examples for

1. handling categorical variables (the output wont be correct until iPFI and iSAGE with categorical inputs #85 is fixed)
2. multiclass problem with numerical features