Implemement a (Deep) Neural Network as engine in ibis

[Martin-Jung]

Among the different families of engines and algorithms in the package, one that we particularly miss at the moment are conventional and/or deep neural networks. Range of packages in R (usually calling python) available that can handle these types of models (e.g. keras or tensorflow).

• Add engine_keras as new engine
• Ensure that all possible data types and options (add_offset(), add_priors(), etc) are supported
• Add some documentation to the package.
• Add a unit testing.