BF - protect intrinsic vol face area against nan
BF - protect intrinsic vol face area against nan

Bad inputs for tetrahedra faces gave nan values

bthirion/nipy,alexis-roche/nipy,nipy/nipy-labs,alexis-roche/nipy,alexis-roche/nipy,alexis-roche/nireg,alexis-roche/niseg,arokem/nipy,arokem/nipy,alexis-roche/register,nipy/nipy-labs,arokem/nipy,bthirion/nipy,bthirion/nipy,alexis-roche/register,alexis-roche/nipy,arokem/nipy,nipy/nireg,bthirion/nipy,alexis-roche/register,alexis-roche/niseg,alexis-roche/nireg,nipy/nireg