New entity "study"

[yarikoptic]

To be used for

• BIDS-MEGA BEP035

which could also encompass some abominations such as "per-archive" compilations such as https://github.com/OpenNeuroStudies/OpenNeuroStudies/ now that we have BIDS "study" DatasetType's, which could potentially join multiple studies together.

attn @bids-standard/bep035