Ivis

This release addresses 50 by making it easy to alternate between CPU- and GPU-enabled tensor flow backend

Major features:

* Support for semi-supervised dimensionality reduction
* Switch from using fit_generator to fit for training the Keras model
* Address eager execution issues with TF 2.0
* User-configurable on-disk-building of Annoy index.
* Tidy handling of interrupted multi-thread processes

Minor features:

* Tests for semi-supervised DR
* Improved input validation
* Better hyper parameter validation
* Slight changes to default hyperparameters

* Bug fixes

* Control eager execution
* R package updates and improvements
* Save ivis object with a custom model
* Bug squashes and performance improvements

Added support for supervised multi-label dimensionality reduction.