NN model for tensorflow 2.3.1

Trained a neural network on tensorflow 2.3.1 with training images from release v0.4.

NN model trained on more grayscale images

• More chessboard training images
  • 390 auto-generated images
  • 126 manually-created based on classifier mistakes

Updated NN model with black-and-white PDF images

• Training images include more manually-verified black-and-white chessboard images from PDFs
• Updated NN model uses 29,248 training images

Trained model on more board types

• Updated nn-model to better detect Lichess images and book diagrams
• Added more training images

Initial neural network model and training images

• nn-model.zip contains a trained tensorflow 2 CNN model that can be used with recognize.py
• training-images.zip contains all images used to train this version of the neural network