Limit the range of extracted corridor featurelines

[Neilw_05]

Often we need to extract featurelines from corridors for grading purposes, e.g. the back of sidewalk for lot grading. If the lot pad ends before the sidewalk ends, we get a featureline that extends beyond the pad, which we don't want. This also happens at intersections where the extracted featureline runs through the intersection because the sidewalk stops and starts on either side.

The workarounds for this are cumbersome and add complexity to the already complex corridors.

Can we make featurelines not span across regions? Or maybe a way to control where they start and stop? This would be at least some improvement to the nearly crippled grading capabilities of C3D.