Validation

Basic functionalities are added.

• Payload validation for both JSON and XML data is added via /validators/
• Working REST-API endpoint added as a flask app
• Error handling is added for the API errors returned to the API caller
• Conversion of payloads to NGSI-LD format added for some of the json payloads

TODO:

• Return unique URI for saved NGSI in the broker
• Finish conversion for JSON and add converter for XML data