N2v

- Add N2V2
- Lifted dependency requirements for compatibility with recent CSBDeep and TensorFLow versions
- Fixed tests

Fix callback import.

* Move from TensorFlow 1 to TensorFlow 2.
* Add test-time augmentation (tta) to prediction. By default it is turned off.

__Note:__ TensorFlow 1 is not supported anymore.

* Single Net per Channel Option
* StructN2V
* ModelZoo Export

Add new functionality:
* Single U-Net per Channel
* StructN2V
* BioImage ModelZoo Export

This releases fixes bug 73