Project monitor incorrectly pads gain matrix

When providing a region level gain matrix for subcortical and cortical sources, the projection matrix computes analytic gain submatrix for the subcortical source space. This causes a shape mismatch.

Solving this is not so easy, since gain matrix for simulation of subcortical sources is still an open problem.

A related issue reported by Paul Triebkorn is that when providing a precomputed gain matrix which includes the subcortical régions, the analytic subcortical gain is added on, causing a shape error as well.