filer_modally should traverse array containers

[inducer]

I think the connections are OK, but I suspect these two might not do the right thing yet:

mirgecom/mirgecom/filter.py

Lines 209 to 211 in 08f191a

	field = modal_map(field)
	field = apply_spectral_filter(actx, field, discr, cutoff,
	mode_resp_func)

When that's done, we can remove the spurious .join() here:

mirgecom/test/test_filter.py

Line 189 in eaa9e6c

uniform_soln = initr(t=0, x_vec=nodes).join()

Here too:

mirgecom/test/test_euler.py

Line 797 in eaa9e6c

dim, filter_modally(discr, "vol", cutoff, frfunc, cv.join())

cc @MTCam @thomasgibson