`len(AFQDataset.sessions)` is not equal to `len(AFQDataset.subjects`

[arokem]

For datasets with multiple sessions per subject, I would expect that the length of the AFQDataset.sessions would be the same as the length of AFQDataset.subjects and that both of these would be the same as the number of rows in AFQDataset.X. However, currently, this is the way that AFQData.sessions is initialized:

https://github.com/richford/AFQ-Insight/blob/cc6e35cb1e46c76cb91e481bf29da683dffefcb0/afqinsight/datasets.py#L222

which means that it has the length of the original data (in the long format).

[arokem]

This is for the case the default case, where concat_subject_session = False

[arokem]

Maybe we need to do something similar to this with the sessions as well?

https://github.com/richford/AFQ-Insight/blob/cc6e35cb1e46c76cb91e481bf29da683dffefcb0/afqinsight/transform.py#L103-L105