ENH: Computation of multi-stream and density field

This commit adds a new class, ProbeVirtualGrid, which extends
the functionality of the VirtualGrid class for storage and
computation of the multi-stream and density fields on the
nodes of the virtual grid, which is typically a refined
version of the initial lagrangian mesh.
BUGFIX: Convert ijk to global extent in FindCell
BUGFIX: Use local coords for VirtualGrid::GetNode
ENH: Implementation of stream-count utility