Use custom RPC mechanism
Rename rpyc_classic_stdio.py to _libmadx_rpc.py

The module is to become a specialized frontend wrapper for libmadx.py.
Fix serious performance issues with pickle.dump
Always use reopened stream objects for Connection

This seems to fix issues that sometimes occur on win32.
Update and improve documentation in RPC module
Use absolute import for libmadx

For the sake of python3 compatibility.
Merge branch 'custom_rpc' into redesign-cpymad-backend