Introduce the new Digital Rock Physics plugin

This plugin introduces two new filters to help the analysis of digital rock.

Analyze Material Clusters filter produces a multiblock dataset that contains two blocks: i/ the second block is a table that contains the volume (number of cells of every cluster) and barycenter of every cluster (set of connected cells that have the same point data material attribute) ; ii/ the first block is the input data image with new point data arrays that correspond to the volume and center of mass of the material cluster it belongs to.

Explode Material Clusters filter creates an exploded surface mesh of the clusters (set of connected cells that have the same material attribute) described in the input image data.

Note that those two filters have two levels of parallelization: they take benefit of data parallelism if it is enabled (eg. MPI), but also take benefit from task parallelism using the SMP feature of VTK if enabled (OpenMP, TBB, etc.) to perform faster.