Support for decay descriptors

[admorris]

As part of the LHCb Ntuple Wizard project, we have implemented decay descriptor parsing (and rendering) using pyparsing. I would like to port this over to decaylanguage and make it easy to add different "grammars" used by different experiments/software packages.

Example grammars:

• LHCb's LoKi decay descriptors
• Belle II's DecayDescriptor class
• KFParticle, used in sPHENIX

We can also think about doing matching and substitution (which we also have functionality for in the Ntuple Wizard) but there it's less clear to me how easy it is to support different conventions from different experiments/packages.