Add SameDiff Layer

[AlexDBlack]

WIP DO NOT MERGE
Still a long way to go here. Needs lots of testing.

Adds SameDiff layer. In summary, to add a SameDiff layer:

• Users extend BaseSameDiffLayer (i.e., the config class), which has methods to return the graph, parameter names and shapes, etc.
• Unlike 'vanilla' DL4J/ND4J layers: users will not usually need to implement a Layer implementation class or a parameter initializer (just the config class above)

For now, I'm focusing on the single layer only case (i.e., no building large SameDiff graph across multiple layers). However, I'm keeping this in mind with the API so it can be added later.

[AlexDBlack]

Closing in favor of: #4675