Add fiducial to parse_request and make_response in tomaat/server/service.py

[faustomilletari]

We have now changed the code to parse requests and make responses without the need for the user to write these functions manually! it is automatic and very convenient. this means we need to add support for types of messages that have been only recently added to the slicer plugin.

@j-frei I hope you can have a look!