Add optional flag to force checkvalbuf_real to use relative instead of absolute tolerances.
Require tolerance of 1e-15 for ieos=12 (ideal gas+rad) forward-backward test. Previous tolerance was 1e-12.
Tighter tolerance of ieos=20 (gas+rad+rec) temperature solving from 1e-13 to 1e-15. Adjust tolerance for forward-backward test for ieos=20 from 1e-12 to 1e-14 (it does not have an error < 1e-15 and I don't know why).