PPIC

This merge adds the following features:

• add the new code LyaPhotonsFromGas (from Jeremy Blaizot)
• faster sampling method in PhotonsFromStars (thanks to Harley Katz)
• no longer use pre-computed PhotTables in Python; instead, fluxes, luminosities, and weights are computed directly in Fortran from SSP/SED library (new module_ssp_lib.f90)
• OpenMP reader for star particles

NB: this branch will remain open to finish the development of the "power law" case in PFS.