add script that produces output suitable for MARLEY

[sybenzvi]

Request from @schol: add a script that can transform output from snewpy into a JSON format readable by MARLEY, the DUNE LiAr event generator.

Notes:

1. This will not be a core part of snewpy but we can add a script to the scripts folder.
2. In principle this script might be better off migrating to MARLEY in the future.

[JostMigenda]

Since this was mentioned again at today’s meeting, I just had a quick look through the MARLEY docs.
The input format expected by MARLEY is documented in section 6.5 of arXiv:2101.11867. It looks like the histogram or grid source types are the most general and would work for any SNEWPY models.

Like SNOwGLoBES, it looks like MARLEY doesn’t have any concept of time, so an approach similar to the initial one in snewpy.snowglobes (generate fluences for separate time bins, write to separate output files, then run SNOwGLoBES/MARLEY on each file) is probably necessary.
Additionally, the JSON output should also include the executable_settings.events key to ensure appropriate normalization across different time bins.

[jpkneller]

It should not be too hard to modify the generate_* functions from snewpy.snowglobes to make the files. The names for the flavors in Marley are slightly different but that can be handled with a little function in the classes in neutrino.py e.g. for ThreeFlavor we add a method:

def to_marley(self):
    name = "v"
    if 'E' in self.name:
        name += "e"
    elif 'MU' in self.name:
        name += "u"
    else:
        name += "t"
    if 'BAR' in self.name:
        name += "bar"

    return name