Spatialparams folder

The spatial parameter classes are located under material/spatialparams. I think this historically may have made sense when only porous media were considered. However, the spatial parameters are more general that. With parameters like gravity and extrusionFactor (see #1001 (closed)) that are not necessarily connected to some material, we interpret the spatial parameters really just as (potentially) spatially varying parameter fields.

What would be a better place? common/spatialparams? Or just move the headers to the respective subfolders, i.e. porousmediumflow spatial params to porousmediumflow and porenetwork spatial params to porenetwork and so on?