SKIPLUS-1012: refactor servers and introduce dispatching server

[tuxalp]

This PR introduces:

• a Refactoring for the servers
• dispatching ability for OJP1 and OJP2 (server.py)

Aim of the refactoring:
The aim of the refactoring is to

• reduce the amount of code redundancy
• improve readability of the code (e.g. not using extensive if/else code blocks for error handling)
• improve and centralize error handling
• allow separation of code for OJP1 and OJP2

Dispatching:

• server.py now dispatches TripFareRequests to according services (api.ojp1.OjpFareServiceOjp1 and/or api.ojp1.OjpFareServiceOjp2), depending on the version flag ("1.0" or "2.0") in the payload.
• Parsing of the version number is implemented in OjpVersionParser.py.
• server_ojp2.py serves ojp2 requests only (like before)
• server_ojp1.py serves ojp1 requests only (new)
• server implementations delegate requests to api.ojp1.OjpFareServiceOjp1 and/or api.ojp1.OjpFareServiceOjp2

The api package:

• the idea of the new "api" package is to introduce an API layer (presentation layer), responsible for the implementation of the OjpFare API.
• includes Error Handling, OJP Version Parsing, services for ojp1 and ojp2
• try to separate api code from other code, e.g. the code responsible for the NOVA client, mapping code etc.

Error handling:

• api.errors: Implementation of Exceptions (e.g. InternalServerError) that can be handled by the ErrorHandler
• ErrorHandler.py:
• ErrorHandler: handles an ApiError exception and creates an according Response using a ErrorResponseContentProvider
• ErrorResponseContentProvider: Creates error response content from a message (depending on OJP version)

[ue71603]

Pls provide details about the refactoring in the description

[ue71603]

@tuxalp Ich möchte gerne, dass Du in der Description zusammenfasst, was das Refactoring macht. Ich würde auch noch warten, bis wir auch das Problem mit Europaplatz gelöst haben. Dann werde ich reviewen.