Better support for splines

[Neilw_05]

When laying out landscape walls, foot trails, drain swales and other features that don't require precise geometry, splines are an efficient tool to get the job done. Trying to model such features with featurelines or alignments requires laying out precise geometry which is tedious and unnecessary. We just need an approximate pretty 3D picture.

I wish we could use splines for site grading, not just as contours. This would include being able to create offset splines, not the multi-segmented polylines we get now when offsetting splines.