bugfix - deal with incorrect assumption that the input data has a transform matrix (for isotropic voxel detection), fixing failing regression function test_bug1527.m

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8710 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0