Use SpatialConstant naming in get_pde_as_diff_op

[alexfikl]

This came up in #279 because the Fourier symbol was derived from the expression returned by get_pde_as_diff_op, but the kernel itself (e.g. the global scaling) used the SpatialConstant naming and conflicted (i.e. could not be simplified).

This seems like a good idea to keep it consistent?

[alexfikl]

@inducer From a quick grep, it seems like get_bessel_arg_scaling in here is the only place that doesn't go through SpatialConstant for the symbol name. Should I change that as well?

sumpy/sumpy/expansion/local.py

Lines 675 to 680 in 5c08eb9

	def get_bessel_arg_scaling(self) -> sym.Expr:
	from sumpy.kernel import HelmholtzKernel
	kernel = self.kernel.get_base_kernel()
	assert isinstance(kernel, HelmholtzKernel)
	return sym.Symbol(kernel.helmholtz_k_name)

[inducer]

Yes, let's that make that a SpatialConstant as well.