bugfix - changed the channel type for BTI reference channel to 'ref' to be consistent with the other systems and made sure units are assigned properly see http://bugzilla.fcdonders.nl/show_bug.cgi?id=2702.

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@9833 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0