Contact roles with adjustments on schemas, tests & default community

Closes #49 (closed)

This MR is based on !54 (closed)

In order to fix some if oddities that we have with the current ${entity}_community_serializer classes, I have serveral changes:

• the root is now always allow_none=False,required=True, as there can't be a community entry without a root term for that it should be used.
• same for the community (we can't have an unit community entry (for example) without having the community given for this entry).
• I added tests for the serializers
• I also realized that there was a problem with the MeasuredQuantityCommunitySerializer (wrong model for the sampling medium).
• I switched over to singular for all the contact role django classes
• I merged the contact roles fixture with the initial data & added default community entries for all of them
• I cleaned up the default community & the script to create it. (it now doesn't contain the measuredquantityunit entries anymore).