API to access allowed values for an input

[nsmith-]

When using a correction, in particular for systematic string inputs, it would be useful to be able to enumerate the allowed values to automate the processing of systematic uncertainties. This would be an evaluator call.

At present, from the schema, one can access such info through the summary method:

import correctionlib.schemav2 as cs
cset = cs.CorrectionSet.parse_file("jectest.json")
nodecounts, input_summaries = cset.corrections[1].summary()
print(input_summaries["systematic"].values)

would return {'nom', 'up', 'down'}

Both this current method and a future evaluator call would need to be modified for #5

[swertz]

This would be very nice; also getting access to the list of correction names in a correctionSet would be useful (e.g., at the moment the JERC uncertainties are implemented as different correction objects). It should be easy enough to give access to the keys of the maps in the CorrectionSet object.