Change-Id: I3b15ae34c0957a0f4026e1886c92a9770e9d170e

Change-Id: I5b174bba02856ede788dcb51ec9b0d598ea7d0df

Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

Updates the API documentation of groups with infos to return codes. The function calls
in the groups API have updated documentation and return `400 Bad Request` status code
if a required attribute is missing.