Inconsistency between projection onto grid and edge intersection

[chrisnatali]

Projection uses spherical calculations whereas edge intersection test uses euclidean space.

For now we use a tolerance in the edge intersection test that suffices on our scale, but it would be best to perform the projection and intersection test using the same math.

[chrisnatali]

"Projection" here refers to the projection of demand nodes onto existing grid done in geograph project_onto. This CAN use a spherical space, but no longer does by default.

"edge intersection" refers to the test to detect whether a candidate edge geometrically intersects with a previously added edge in the modified minimum spanning tree algorithm (geometric intersection is considered a new connection which introduces a cycle...and we can't have cycles). This is called in mod_boruvka here.