Abundances T_start and T_end could track thermo by default

[cgiovanetti]

Currently both can be specified and both default to const.T_start/const.T_end. If adjusted in thermo, would be nice if AbundanceModel detected and adjusted for the range it was given

[cgiovanetti]

Would be even nicer if the thermo module returned an object I could pass into abundance model, rather than all of these separate arrays