remove local copy of BIDS specs

At the moment, in BIDSTools/resources, we have a copy of the BIDS specs... Everywhere this local info is used in the code, it should be removed and replaced with an extraction from the online BIDS specifications (somewhere in here: https://github.com/bids-standard )

This concerns BIDSDatatype.py and other files (probably most files where the filename starts with BIDSXXXX.py... to be checked)

