General Notes
Abstract classes are not instantiated
All mappings must start with Transmittal_Root
Aggregations are used to group data by some attribute (date, time, location, etc.), i.e. base forecast time, season, etc.
Property_Grids must have at least 1 spatial axis
Pressure levels and sigma surfaces are not spatial dimensions
Property_Grids & Property_Tables can contain other Property_Tables
Points can have Property_Tables and Property_Values
Table_Property_Description describes what is contained in table cell