This is a translation of Sebastien Morin's C code in the old relax_disp branch. The old C files can
be checked out with the command:
svn co http://svn.gna.org/svn/relax/branches/relax_disp/maths_fns/dispersion.c@r18123
svn co http://svn.gna.org/svn/relax/branches/relax_disp/maths_fns/dispersion.h@r18123
