[Help-gsl] contribute code for Wishart distribution

[Timothée Flutre]

Hello,

I wrote some code to sample random matrices from the Wishart distribution,
as well as compute the (log) probability density function, along with tests.

For that I also added some code to cholesky.c to solve AX=B (where X is a
matrix) where A is provided via its Cholesky factor (but I didn't wrote any
test for that as I don't really understand the existing tests.)

Patrick, can I send you the different modified files directly?

Best,
Timothée Flutre