Swap necessary offers and requests for the case of choreography

[davidebasile]

[davidebasile]

the mapping from contract automata to communicating machines was mapping offers to outputs and requests to input.
Thus when introducing modalities, offers were rendered necessary whilst requests were always permitted.
This is the opposite case of orchestrations and further introduced complexity due to this duality.
This could be solved by mapping offers to inputs and requests to outputs in the mapping.