Improved treatment of IGM absorption

This release contains a significant improvement to the treatment of IGM absorption, which has a critical effect on the rest-frame extreme-UV fluxes (<0.12um) of z>4 galaxies. In the previous versions of the code, the absorption was fixed to a uniform value for all galaxies, as it was baked into the template library and could not vary. This was mostly fine for galaxies at intermediate redshifts, but it was producing too much extreme-UV flux for high-z galaxies. This is now fixed: each galaxy has its own estimate of the IGM absorption, which depends on the redshift according to the Madau+95 prescription (as in FAST).

This change degrades performances only slightly and is thus made the new default. Still, if you do not care about the IGM absorption or high-z galaxies, you can recover the previous behavior of the code using the naive_igm flag on the command line.