Feature: Rewrite NiftyNet to use TensorFlow 2.0

[snorrealm]

It would be very neat to have NiftyNet use TensorFlow 2.0 instead of 1.x. This especially for our use case where we want to speed up the training time by using the better TF 2.0 multi-GPU support.

[tvercaut]

As discussed in #484 NiftyNet is not actively mainatained anymore. Most of the NiftyNet development efforts have now been redirected towards MONAI.