allow layout to index files in the derivatives folder a raw dataset

[Remi-Gau]

This is a valid BIDS directory structure, so it should be possible /convenient to index the content of some of those extra folders:

tests/bids-examples/ds000117/
├── derivatives
│   ├── freesurfer
│   └── meg_derivatives
├── stimuli
│   ├── func
│   └── meg
├── sub-01
├── sub-02
...
└── sub-emptyroom

[Remi-Gau]

These files should also be potentially copied to a new derivatives when copying the raw data: see #171 (comment)