diskpn: accretion disk, black hole, black body

Blackbody spectrum of an accretion disk. This is an extension of the
diskbb model, including corrections for temperature distribution near
the black hole. The temperature distribution was calculated in
Paczynski-Wiita pseudo-Newtonian potential. An accretion rate can be
computed from the maximum temperature found. For details see
Gierlinski et al. (1999). Please note that the inner
disk radius (`par2`) can be a free parameter only close to `par2` = 6;
otherwise `par2` is strongly correlated with `K`.

par1 | maximum temperature in the disk (keV) |

par2 | inner disk radius in Rg = GM/c units,
par2 |

norm | , where = central mass (solar masses), distance to the source (kpc), inclination of the disk, and color/effective temperature ratio. |