Rest API Implementation

[PhilMFischer]

For the Virtual Satellite Rest Server the external API is needed as well. It should provide various levels of access to Virtual Satellite data. The following levels need to be implemented:

• [x] Server Management level to add or configure repositories (Implemented in #540)
• [x] Low Level Data API to access StructuralElementInstances and CategoryAssignments
• [ ] High Level Data API, to e.g. Activate a Concept, run Apps or Calculations, Run IO WIzzards
• [x] Low Level Data API for managing Users
• [x] Optional Low Level API for Managing Units.

The APIs should follow the URL pattern of http://localhost:port/rest/[API]/[VERSION]/... .

REST API functionality should follow the general API conventions. Primary Data Source is JSON, XML is not considered for rthe moment.

Replaces #361 and Closes #361.

[SaMuellerDLR]

Check code from #361 for some previous implementation