Test data of composite experiments

[yaelbh]

CompositeExperimentData does some complex stuff, especially when it comes to parallel experiments. It takes the results of the composed circuits execution, marginalizes results for each sub-experiment, and stores them in the experiment data objects of the sub-experiments.

Currently we don't have unit tests that directly test this process.

[yaelbh]

I think #380 adds tests along these lines, therefore I suggest to wait for #380 and then evaluate if more tests are needed.

#437 may also be related, although I think it's independent.

[yaelbh]

I don't see something like this in #380, so the work on this issue is independent of #380

[yaelbh]

Done in #506