Upgrade n3fit to Tensorflow 2.0 (and Keras 2.3)

Created by: scarlehoff

Tensorflow 2.0 will be released soon (with many new improvements). A new backend should be added where the changes to make it compatible with Tensorflow 2.0 are implemented.

Since the latest release of Tensorflow (1.15) is now forward compatible with 2.0, once 1.15 is in conda-forge both backends can be merged.