optimizers & hyperbolic funcs

[meiji163]

• Moved hyperbolic functions to utils.math. When working with the library it was inconvenient having to access functions through the "Poincare" class. The benefit of having the class isn't clear to me.
• Fixed some errors in the hyperbolic functions and added a few functions (hyp_dist, clipped_norm, parallel_transport, gyr, lambda_x)
• Rewrote RSGD. Optimizers should inherit from keras optimizer_v2 and implement sparse and dense updates separately (see here). It should be possible to momentum next.
• attempted RAdam, still buggy. The problem is parallel transport of momentum ( see Becigneul & Ganea pg. 5 )
• Added Jupyter notebook in /examples to demo word embedding. Not sure where to put this but it could be nice to have more demos/tutorials in the future.