Raise polyline to surface

#1706 

`PolylineGeometry` and `SimplePolylineGeometry` are raised to the surface of the ellipsoid by default.  To get a straight line, set `options.raiseToSurface=false`.

Also cleaned up functions in `PolylinePipeline` and tweaked sandcastle examples.
This doc is the same as `generateArc`. Could you add to the description of both something about the return type? The code example needs to be updated here as well.