Expose training code of the Facemark API for python

[shardulparab97]

I believe that exposing the Facemark APIs training section for python would be helpful for python users and help OpenCV grow as a go-to library for face recognition purposes.

[shardulparab97]

This initial commit is an attempt made a few weeks ago for exposing the training functionality of Kazemi method. It would be great to hear a review on the same.
However, this is yet not an ideal solution, since there is shortwiring done at places.

Herein I had to set in my own default detector - in calcMeanShape - because otherwise we need to set the detector using setFaceDetector - we need to do refactoring to bring in this function out to python/ search for some other way out.

This code is very preliminary, simply giving a start to solve the given issue and would like to hear reviews to begin refeactoring as soon as possible. 😃

[LaurentBerger]

@shardulparab97 @alalek I need Facemark API in python. I done some jobs here. I've seen another PR about facemark in python .
I can push my own job but I think it will make things complicated