diskm: accretion disk with gas pressure viscosity

A disk model with gas pressure viscosity. The spectrum from an accretion disk where the viscosity scales as the gas pressure. From Stella and Rosner (1984).

par1 accretion rate in Eddington Luminosities
par2 central mass in solar mass units
par3 inner disk radius in gravitational (= 3 Schwarzschild radii)
par4 viscosity
norm $2 \cos(i)/d^2$ where $i$ is the inclination of the disk and $d$ is the distance in units of 10 kpc