Improve bin handling for discrete parameter types

Currently there is no way to pre-inform carpenter what values to expect for discrete variables, eg. categoricals, natural numbers etc. This results in bins that would be expected, not being present in the resulting dataframe, especially for natural number variables. The config file specification should be expanded to handle these situations better.

First raised within CMS-CHIP: https://gitlab.cern.ch/cms-chip/chip/issues/7