PyProcessBuilder: clean up Ref-/VarPort initialization

[PhilippPlank]

The initialization of RefPorts and VarPorts during build(..) in the PyProcessBuilder expects 2 csp_ports, one receiving and one sending.
Currently there could be a list of csp_ports. This needs to be cleaned up.

Should be done in parallel with Issue #55

Reference:
This is not so elegant and looks like a hack. Normally there could be multiple csp_ports in case of 1:many or many:1 connectivity.
Here there are multiple ports because we've just misused it to pass the different send and recv ports. That's conceptually different. If we throw errors that forbid non 1:1 connections then this will work but we should find a better solution.

Either implement a better way of file a descriptive issue to address this hack and make it cleaner.

Originally posted by @awintel in #46 (comment)