disc algorithm location

[wholmgren]

The disc algorithm is currently in clearsky, but I think irradiance is a better place for it (and the rest of the similar models that still need to be implemented #2 #4).

[wholmgren]

Thoughts @bmu @uvchik @Calama-Consulting? Would like to resolve this before we add some of the other matlab functions.

[bmu]

Should be moved, I think.