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