Testing version

Tests for Brownian displacements.
Testing version.

This version is messy, as it includes 3 ways to compute Brownian motion.
Those 3 ways seem to work equally well.
It has multiple testing lines, commented or not.

This version is kept only for future tests if needed.
New implementation of Brownian motion.

Removed implementation a la Banchio/Brady.
Reimplemented previous Cholmod-based implementation, BUT correcting
an important bug on the way.

Previous implementations forgot about permutations associated with
the Cholesky factorization provided by cholmod, leading to an
erroneous computation of the Brownian displacement.

This is now corrected, and the current version seems to give much
more reasonable results, while being much faster than Banchio/Brady
algorithm.