bugfix - don't try to convert the channel units between mm/cm/m if they are unknown


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8698 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0
automatically synchronized identical files to revision 8698

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8699 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0