Filter datasets by characteristics

[arteymix]

We will likely need an endpoint in Gemma to retrieve and organize available characteristics.

• [x] simplify filter by whole category by using a category URI
• [ ] consider simplifying an "everything but term A, B and C in a category" using a "not in" operator. This operator does not exist yet however.

[arteymix]

We now have all the necessary backend support for navigating term hierarchy and filtering by terms.