Cosmit: Syntax simplification in multiple_fast_inv.
Cosmit: Syntax simplification in multiple_fast_inv.

This helps better understanding what is the code logic.
I do not think it can be further factorized yet.

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