Write a C++ implementation of the interfaces

[michaelosthege]

The metadata classes can be auto-generated from the protobuf.
What's needed:

• Package structure
• Including a C++ protobuf compile step in scripts/generate.py:
  1. protoc -I=mcbackend/protobufs --cpp_out=$DST_DIR mcbackend/protobufs/meta.proto
  2. Move the generated files to the right place in the package structure
• Interfaces or base classes for Backend, Run and Chain
• Helper functions to convert between Ndarray ProtocolBuffer objects and C++ arrays (the Ndarray().data must be assigned bytes data in same encoding used by NumPy)