[BF] Fix bug in multiple_fast_inv.

Some computations were done outside the loop, but they depend
on values changing at each iteration.
Add test for multiple_fast_inv.

That test would fail with earlier versions of the routine.
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.
Merge pull request #249 from VirgileFritsch/master

BF: Fix bug in multiple_fast_inv

Some computations were done outside the loop, but they depend on values changing at each iteration. This resulted in wrong inverse values because of the use of the wrong piv variable. Also the check of info was done only for the last decomposition.