SDM refactor training
SDM refactor training

Move training into the constructor. This makes it easier
to serialize SDMs. Now, you build a single model which gets
trained inside the constructor. Therefore, I also refactored
the train_incrementally function to inside the train method -
so all training can be done incrementally.

grigorisg9gr/menpofit,yuxiang-zhou/menpofit,yuxiang-zhou/menpofit,grigorisg9gr/menpofit