Create a method to pad the projection matrix with zeros when subcortical structures are present.

Not all the methods for calculating the projection matrix operate on
collection of dipoles. Many of them operate on a surface. So, in many cases
the lead field matrices are of dimensions number_of_vertices x number_of_sensors

A solution is to pad the projection matrix with zeros for any non-cortical
regions.

Maybe this step should be done when the simulator is configured.