add MIYAMOTONAGAI, currently only equivalent to a Plummer !!! Change 1e5/3.085e+22 by HUBBLE in init_outer_potential_nfw Change 1.989e33 by SOLAR_MASS in init_outer_potential_plummer
git-svn-id: https://svn.epfl.ch/svn/gasp@9 67b3a02f-582b-4e65-a7f6-2b87bd0cff7a