add Mori2020 model

[JostMigenda]

Mori-san recently contacted me about adding the Mori2020 model to SNEWPY. It uses the same format as the Supernova Neutrino Database. Previously, we’ve converted those files to a different format assuming a pinched spectrum (@nuberoi, is the script to do this available publicly?); but I would suggest implementing the spectral interpolation described by Nakazato for improved accuracy.[1] I’ve already implemented that for sntools, so this shouldn’t be too much extra work; but don’t know yet when I’ll find the time.

[1] Comparison of original interpolation with pinched spectrum:
Running sntools <input file/format> --detector HyperK --distance 50 --starttime 100 --endtime 300 for both nakazato-shen-z0.02-t_rev100ms-s20.0.fits (SNEWPY’s fit to a pinched spectrum) and intp2001.data (original file, sntools’ implementation of the spectrum interpolation described by Nakazato), I get the following number of expected events:

channel	orig.	pinched
es-e	7.84	7.88
es-eb	3.31	3.33
es-x	2.01	2.02
es-xb	1.73	1.73
ibd	223.49	223.21
o16e	2.59	0.84
o16eb	3.24	2.35

So the difference is negligible for IBD and elastic scattering, but up to a factor of 3× for oxygen-16 CC interactions; presumably because the difference between spectra is only significant in the high-energy tail, which O16 interactions are particularly sensitive too.