[Paraview] new features for the calculator

[Berk Geveci]

>  - a d/dx, d/dy and d/dz operator

vtkCellDerivatives does this. If I find the time, I will enable this
in the ParaView 2.6 filter menu.

>  - a max and min operators

This would be  a good addition.

>  - a coordinate, (x,y,z), field (which exists in Paraview III :)

Andy Cedilnik added this to ParaView 2.6.

We have some long term plans to overhaul the array calculator. We are
also adding support to add python based custom filters to ParaView 3.

-berk