Implement across trials in core-db

This implements support for across trial functionality transparently (no API changes; core-api is unchanged).
It does not support visit based across trial nodes (I don't even know how that works and there is no example data in the test server).

Note that the rest-api still won't be able to use across trial functionality. There are two reasons:

The 'across trials' node is not exposed as a study.
Clinical data API composed variables are not supported for across trial yet. AFAIK, the rest API is the only client that uses composed clinical variables.
It is also not possible to aggregate high dimensional nodes yet. This is due to DB schema complications (de_subject_sample_mapping is linked to a specific concept).