read GENTYPE map from sim-readme

[RickKessler]

regarding SNN updates, I wanted to bring to your attention some sim-bookkeeping updates that I made for the SCONE classifier and that should be useful for SNN as well. Scone and SNN both had lists of integer types to specify SNIa/nonIa, but this information can now be robustly read from SNANA sim's output readme; for example, on RCC scroll down thru
$SCRATCH_SIMDIR/ROMAN_1TIER_PRISM25_DATA-a03-t00-z04/ROMAN_1TIER_PRISM25_DATA-a03-t00-z04.README
and check the GENTYPE_TO_NAME block.
If you scroll down further in the README, there is a FILTERS block giving mean wavelength per band.