add support for BIDS atlases

[robertoostenveld]

BIDS BEP038 is making nice progress and proposes a straight-forwards way of storing atlases. If this gets widely adopted, atlases will become much simpler to use.

• We should implement support for the proposed format (which is a nii.gz and a tsv file) to ft_read_atlas.
• We should document it on http://fieldtriptoolbox.org/template/atlas/
• We could convert the existing atlases that are included in https://github.com/fieldtrip/fieldtrip/tree/master/template/atlas

mentioning @PeerHerholz and @schoffelen

[schoffelen]

This should be quite straightforward given my recent clean-up of ft_read_atlas