BF: fix axis detection error for earlier numpies
BF: fix axis detection error for earlier numpies

numpy 1.2.1 (at least) argsorts nan to the beginning (a low value), but
later versions argsort it to the end (a high value).  Enforce the high
value interpretation by replacing with inf.

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