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 (nested tables).
Points can have Property_Table and Property_Value components.
Table_Property_Description describes what is contained in each table cell.