Make it possible for a derived bioassay to have multiple physical bioassays as parents

[Nicklas Nordborg]

The use case is that a single sample is split on two flow cells. This is for safety reasons if there is a problem during the sequencing that results in losing all data from one of the flow cells. The first steps of the analysis is done per flow cell, but in a later step data from the two flow cells are merged into a single derived bioassay.

Note! This should not be too difficult to implement but probably breaks backwards compatibility in the core API. But this time we'll make an exception to the rule not to break compatibility.

This change is also related to #1708.