[Bug-gsl] Missing constants

[Juho Schultz]

Dear sirs,

Thomson cross section and radiation density constant could be added to
const/gsl_const_mks.h

#define GSL_CONST_MKS_THOMSON_CROSS_SECTION (6.6524585e-29) /* m^2 */
#define GSL_CONTS_MKS_RADIATION_DENSITY_CONSTANT (7.56591e-16) /* J m-3 K-4 */

feel free to cut-and paste these to the sources.

   Juho Schultz

   e-mail: address@hidden
   www.astro.helsinki.fi/~jschultz
   Observatory
   P.O. Box 14
   FIN-00014 University of Helsinki
   FINLAND