Resolve "Add a Model class in OTBTF"

Closes #29

Closes #18 (closed)

Huge improvements in python API

• Add a ModelBase class to implement quickly a deep net, that will work out-of-the-box in TensorflowModelServe application. The model inherits from keras model class, and can be used as such. Provides a very intuitive framework to code:
  • model inputs preprocessing (optional)
  • model implementation (mandatory!)
  • mode output post-processing (optional)
• Harmonize Dataset.dataset and TFRecords classes
  • dataset preprocessing function (optional)
  • dataset splitting (inputs, targets) to be ready to use in keras models
• Some documentation (in the examples/tenso ‎rflow_v2x/fcnn code comments and README)
  • How to create a deep net from scratch
  • How to train the deep net from patches-images
  • How to train the deep net from TFRecords files
  • How to convert patches-images into TFRecords files
• The API is tested in the CI using the T&F CRC BOOK data