Support for Tensorflow 2.0

[jthestness]

Later versions of TF 1.x give warnings about loading models such as the following:

WARNING:tensorflow:From /usr/local/lib/python3.7/site-packages/tensorflow/python/training/saver.py:1266: checkpoint_exists (from tensorflow.python.training.checkpoint_management) is deprecated and will be removed in a future version.
Instructions for updating:
Use standard file APIs to check for files with this prefix.

Tensorflow 2.0 does not support the MetaGraph (.meta) file loading functions as before. We need to update these file loading paths to maintain forward compatibility.