New scheme for two level atom

There are three changes in this pull:

radiation.c : a new subroutine called mean_intensity. This accepts a mode (to be discussed perhaps), a frequency and a cell. It returns J for a given frequency/

lines.c : Changes to two level atom, so it now uses J explicitly, wether a dilute blackbody or a model.

compton.c : Simply calls mean_intensity to get J rather than calculating it internally. Lots of commented lines and one call!