viskores: Add it as a snapshot dependency
Part 1 of a bigger task to remake the VTK dependency in VTK-m(Viskores), part 2 will be moving the existing vtk-m code to Viskores.
In this part we add viskores as a snapshot dependency. VTK-m has historically been a git submodule of VTK due to the strong relation between the two projects. Whereas the relation remains, there is little rationale to keep this sort of dependency for newer dependencies and we now know have a clear and standardize way of including thirdparty dependency in VTK.
This will also help building VTK with external Viskores (specially important in the Spack context).
Note that there is not symbol mangling in Viskores as there was not in VTK-m.
Also note that while there is not existing code in VTK using this library, we will began the transition process of VTK-m code right after this gets merged. We figured that it will be easier and cleaner for devs and reviewers to divide the transition task in smaller units with the idea it might take longer to complete and that we could have multiple people in parallel working on this.