parcel_model: Modernization and updates for Python 2/3 compatibility

This release denotes an overhaul of the model documentation and modernization of its codebase, instigated on the release of Rothenberg and Wang (2016). The scientific description of the model in that article is still accurate and is reflected in this release of the code, but the model has been processed to update documentation where out-of-date and to enable compatibility between Python 2 and Python 3