Rest API: create OSME musicXML from given parameters

[matt-uib]

e.g. give me 8 measures in D-major with difficulty 5...

shall work on e.g. the PracticeBird Server where OSMD and OSME is installed:

• handle input parameters
• calculate object model with OSMD/E
• export xml file
• send back to requester.

[FloHin]

Done, see MR#2 for this