Changes for Proga release

Main changes:

Significant rewrites on the PL and EXP representations of spectra in cells. 
PL now uses a log representation and EXP can have -ve temperature (i.e. a rising exponential). The min and max frequencies of photons seen in a cell/band are now logged, and these are used in the variable temperature code as limits of integration for models. 
Versions of IP and J have been added which log the component from direct and scattered radiation.