Make a kernel PDE getter that returns the system PDE #284

Right now, Kernel.get_pde_as_diff_op returns a PDE satisfied by each component. For example, the Stokeslet returns the biharmonic:

Maybe that should be renamed get_scalar_pde_as_diff_op. And then get_system_pde_as_diff_op could return the system PDE (i.e., Stokes).

cc @alexfikl