flatten class hierarchy: EdgeWithElevation -> StreetEdge <- PlainStreetEdge

We may want to pull the elevation field back out into a _sub_class of StreetEdge
in order to save memory on some deployments without elevation data (NL)
replace wheelchair accessible boolean with a flag
remove edge labels, which don't seem to be used anywhere
rename PartialPlainStreetEdge to PartialStreetEdge
store edge length as 32-bit fixed point millimeters. remove redundant method.
remove unused vertex group numbers
correct method parameter names (lat,lon should be x,y)
store in and out angles as single bytes. add comments explaining range and reference angles.