Abstract base class for trainable facemark models
 To utilize this API in your program, please take a look at the REF: tutorial_table_of_content_facemark
 ### Description
 The AAM and LBF facemark models in OpenCV are derived from the abstract base class FacemarkTrain, which
 provides a unified access to those facemark algorithms in OpenCV.
 Here is an example on how to declare facemark algorithm:
 
 // Using Facemark in your code:
 Ptr<Facemark> facemark = FacemarkLBF::create();
 
 The typical pipeline for facemark detection is listed as follows:
 
   - 
  (Non-mandatory) Set a user defined face detection using FacemarkTrain::setFaceDetector.
   The facemark algorithms are desgined to fit the facial points into a face.
   Therefore, the face information should be provided to the facemark algorithm.
   Some algorithms might provides a default face recognition function.
   However, the users might prefer to use their own face detector to obtains the best possible detection result.
   
- 
  (Non-mandatory) Training the model for a specific algorithm using FacemarkTrain::training.
   In this case, the model should be automatically saved by the algorithm.
   If the user already have a trained model, then this part can be omitted.
   
- 
  Load the trained model using Facemark::loadModel.
   
- 
  Perform the fitting via the Facemark::fit.