Model phenotypic information at session level

[x] Create two new session subclasses, for imaging and phenotypic sessions
- https://github.com/neurobagel/bagel-cli/blob/4da00b6db4cce30d40f101c0c4e17be25db3828f/bagel/models.py#L52-L56
[x] Move currently subject-level attributes under the phenotypic session
[x] Add examples for a case where there is no phenotypic session column in the input TSV
[x] Handle creation of new phenotypic session when there's no session column but there is phenotypic data
[x] Add validation for duplicate session IDs
- [x] add a test that uses example1 (currently not tested against)
- [x] update input validation utils
[x] Add validation for duplicate subject IDs too! Cause we forgot to add this OOPS
- [x] add some new examples for this
[x] Update existing tests
[x] Remove leftover TODOs

neurobagel / bagel-cli