Multi Measurement Initiator - Error on MultipleHypothesis Associations

[ztarsasw]

Problem Description:
The initiate method of MultiMeasurementInitiator currently assumes that the associations provided by the initiator's data_associator will be SingleHypothesis types. However, this will error when the user configures the initiator to use a hypothesiser that returns MultipleHypothesis types as it does not define measurement or prediction members.

Steps to Reproduce:
Define a tracker with a MultiMeasurementInitiator that is using a PDAHypothesiser within it's data_associator.

Desired Solution:

1. Ideally, the MultiMeasurementInitiator should be able to use any valid hypothesiser/data_associator combination, not just ones that can produce SingleHypothesis types.
2. It may not make sense for the MultiMeasurementInitiator itself to work in a Multi-Hypothesis context (especially with the releasability criteria being number of detections). At the very least, this should be mentioned in the documentation.
3. In the case of 2, it might make sense to implement an analogous initiator that can specifically release tracks based on some sort of analysis of the MultipleHypothesis type (like how the MixtureTracker's __next__ method is implemented).