Code cleaning.

Removed useless code and useless include to gsl_rng.h in
ut_space/ut_space.c.
Add linear system resolution function.

Added function ut_linalg_solve_LU for solving linear systems, A.X = B,
where A is a square matrix of size n x n and B a vector of size n.  The
function uses the GSL for solving the system.
Add functions ut_num_bound*.
Add ut_space_point_plane_dist_signed.

The function returns the signed distance between a point and a
plane.  The sign comes from the orientation of the normal of the plane.
Add ut_sys_mkdirp.

The function creates directories, recursively.
Modify ut_print_message.

The function now aborts if an error flag is passed.