[spd] = GenerateBlackBody(T,wls_in)
Generate spectral power distributions for black body radiators.
Generated according to formula in W+S, pp. 11-12.
We compute output as radiant exitance in units of W m-2 nm-1.
T - row vector of desired temperaturs in Kelvin.
wls_in - column vector of wavelengths.
spd - the spectral power distributions are in the columns.