SEEREP Python API re-design and extension.

[jarkenau]

While working on the Python examples today, I noticed that we need to do something about the structure of the generated Python files of the gRPC API and our helper functions. It's getting more and more complicated to use them, and a lot of lines could potentially be saved.

My main point would be:

• Change package structure (proposal below)
• Add type annotations Add type annotations to seerep.util scripts.  #297
• Update the package description on PyPi

Since we now also want to start with integration tests in #294, it probably makes sense to do this before and safe the work of redoing the integration tests after a major API change.

Proposed package structure:

• seerep
  • seerep.pb
    • seerep.pb.generated (generated gRPC interface + messages)
    • seerep.pb.msgs (functions which encapsulate the message building process)
    • seerep.pb.grpc (functions which encapsulate gRPC calls)
  • seerep.fb
    • same layout as pb ....
  • seerep. common (code shared between PB and FB, e.g create a gRPC channel)