Fix `size` method for `SequenceDesign`

[jschepers]

Implement/Fix size method for SequenceDesign. Idea: do it similarly to the MultiSubjectDesign where it is n_trials x n_subjects (if I remember correctly) i.e. something like n_trials x n_events _in_sequence.

[jschepers]

@behinger I tested it with a SequenceDesign{SingleSubjectDesign} and it did not work. I assume the function above does not match SequenceDesigns that are wrapped around other Designs (which is always the case I assume). But maybe the problem is elsewhere.
Is there a test for the size method above for SequenceDesign?