Reorganise tags in OpenAPI spec

[cmil]

Tags could better reflect functionality of the API (see #192)

Technically, I think, the groups are "Tags", right? So you can assign multiple tags to an endpoint which would result in making it show up in several of these groupings. IMHO, it would make sense to introduce more of these, e.g. group endpoints that provide the data for Network Analysis,...
I'm all for improving the readability of the OpenAPI documentation by giving it a clear structure. Maybe we should think about replacing the "public" tag by one or more tags that better serve this purpose then.

This issue was originally part of #194.