This fixes some DXFs imports where unforunately CAD tools like SolidWorks
randomly decide to mirror circle definitions across the "z" axis (resulting in x or y axis flips in 2d)
Most likely live projection from 3D to 2D drawings introduces this.
However this is DXF specification to describe it so obtusely with vectors for a 2d drawing.
This only tests the InterceptsPositiveX() and InterceptsNegativeX() used
for finding the midpoint of an arc.
Tidy up some of the trigo.h header Doxygen comments.
Marek Roszko
Tomasz Wlostowski
Ian McInerney
Wayne Stambaugh