Support vector arguments with offset in scan kernels.

The boxtree and pytential test suites issue a lot of warnings related to the fact that the tree build kernels don't support VectorArgs with offsets. Unfortunately the tree build kernels can't easily be converted to support VectorArgs with offsets because of code sharing with elementwise / scan kernels, the latter of which up to now have not supported VectorArgs with offsets. This fixes the scan kernels to support them.