Added the missing mpower() function as lib.linear_algebra.matrix_power.square_matrix_power().
This is needed by the lib.dispersion.ns_2site_star module.
The function comes from the 'fitting_main_kex.py' file attached to comment 3 of task #7712
(https://gna.org/task/?7712#comment3, https://gna.org/support/download.php?file_id=18263).
The mpower() function was copied and modified to suite relax's coding conventions.
