number of products to handle per WPS process

a policy on how many products can be handled in one WPS processes, the current processes have different approaches when comes to that, for example:

• GetStatistics process: inputs are a bbox, and the response is the statistics of all the coverages within (or intersects with) the bbox.

• GetDEMProcessing (contour), GetHeightProfile: one process per coverage, in fact the coverage ID is one of the inputs in both processes