Add tests for advecting particles in explict cell sets

The UnitTestParticleAdvectionFilter was only testing advection in structured cell sets (with either uniform or rectilinear points). This did not test all paths (i.e. grid evaluators and locators) of particle advection. Changed the test to also try grids with explicit cells.

Note that the explicit cells are created by just converting the rectilinear grids to explicit grids. It would be better to get at least one small dataset that starts as unstructured.