Mnx C++ Dom

[rpatters1]

Since no one else admits to having created a C++ DOM for MNX, I went ahead and created one myself. I've been using it for my Finale export. I believe it now includes every object in the current MNX spec, but perhaps there is a corner or two I missed. I hope to keep it up to date as the spec evolves, but I am not averse to help along those lines.

Github repo
Documentation Pages

Features are:

• it wraps around an nlohmann json object that is the root object of the document
• each class is lightweight, containing only a shared pointer to the root json and a json_pointer to its place within the json
• that means no need for class-by-class serialization
• uses macros to generate boilerplate class functions for each mnx property or child object