allow for a non-uniform BC in the helmholtz solver.

[Pedro Costa]

This can be quite important when implicit diffusion is used in conjunction with a non-uniform inflow (e.g. a Poiseuille or a square duct inflow).

Edit: the non-uniform B.C. has to be reflected in:

1. the coefficient matrix
2. the imposition of the B.C. in the right-hand-side of the Helmholtz Eq.