Migrate "subjects" to "sessions"

[kousu]

Same as spine-generic/data-multi-subject#102, but more obvious in this dataset:

every scan here is the same subject, so according to BIDS, should be arranged like this:

.
└── sub-01
    └── sub-01_sessions.tsv
    ├── ses-chiba750/
    ├── ses-chibaIngenia/
    ├── ses-douglas/
    ├── ses-glen/
    ├── ses-juntendo750w/
    └── ses-juntendoAchieva/
    ....

The fields in participants.tsv should be split up so that the constants are in participants.tsv which should only have a single row, and the ones that change per site/scanner are in sub-01_sessions.tsv