ENH: added functionality for smoothing the sensitivity derivatives

A Helmholtz-like filter is applied to the original field of sensitivity
derivatives. The corresponding PDE is solved on the sensitivity patches,
using the finite area infrastructure. A smoothing radius is needed,
which is computed based on the average 'length' of the boundary faces,
if not provided by the user explicitly.

If an faMesh is provided, it will be used; otherwise it will be created
on the fly based on either an faMeshDefinition dictionary in system or
one constructed internally based on the sensitivity patches.