Simplified schema and tweaked some bits

Removed obsolete ContextAmplifiers and SpecialGraphic attributes
Improved documentation and svg READMEs
Changed air corridor symbols to lines
Updated affected csv files
Changed the location of the geometry exception tags
Changed the descriptions on the gct tags
I'm receiving everything.

Your comment here https://github.com/ArcGIS/military-features-pro-data/issues/57#issuecomment-61812704 was confusing, apparently.  Since gct values occur less frequently than any other value in the tag list, I chose to place them at the end, but before the last three tags that you've told me before need to be the last three tags.

They are simple enough to move wherever you want them...