Directory structure around C++ code and custom ops

Right now the implementation of custom ops lives in /operations and the linking python code in /tf-encrypted/operations. This doesn’t seem to match neither https://github.com/tensorflow/tensorflow nor https://github.com/tensorflow/custom-op.

Knowing that we’ll soon see more functionality written in C++ (eg HE) as well as more custom ops (for accessing external functionality and for optimizing protocols otherwise written in python) we should figure out which directory structure works best.

Related #568

[ianlivingstone]

Whichever directory structure we choose, we should ensure that we think through:

• Developer setup and maintenance
• The underlying build system and ensuring it’s easy to understand, use, and maintain
• Having a cohesive strategy for testing the different components and how they interface together
• Clear documentation on how to add an additional operation
• Clear documentation on how all of this is built in CI and results in a published package

[mortendahl]

Closing this up. For now we’ve landed on https://github.com/tf-encrypted/tf-encrypted/issues/564#issuecomment-501713688 and https://github.com/tf-encrypted/tf-encrypted/issues/564#issuecomment-501733882