Make a kernel PDE getter that returns the system PDE

[inducer]

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

sumpy/sumpy/kernel.py

Lines 1023 to 1027 in c3e1a73

	@override
	def get_pde_as_diff_op(self) -> LinearPDESystemOperator:
	from sumpy.expansion.diff_op import laplacian, make_identity_diff_op
	w = make_identity_diff_op(self.dim)
	return laplacian(laplacian(w))

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