Porting external functions (such as `bessel_j` / `bessel_y`) to New World Grudge

[thomasgibson]

A reminder to convert these functions to the revamped Grudge framework. Currently, #74 still calls out to grudge.sym to handle certain external functions. In particular, the Bessel functions.

grudge/test/test_grudge.py

Lines 990 to 994 in a494ba0

	# FIXME: Bessel functions need to brought out of the symbolic
	# layer. Related issue: https://github.com/inducer/grudge/issues/93
	def bessel_j(actx, n, r):
	from grudge import sym, bind
	return bind(dcoll, sym.bessel_j(n, sym.var("r")))(actx, r=r)

[inducer]

We probably want these in the array context, maybe actx.scipy.special.bessel?

[thomasgibson]

That could be really nice. Would we strive to basically have something like this: https://docs.scipy.org/doc/scipy/reference/special.html?

[inducer]

I don't imagine we would make it to more than a small subset. Special functions are way challenging to implement well. To give you a sense, cephes' (an old, well-established library in this space) returns mostly garbage from its Bessel functions if the arguments are any bigger than 30 in absolute value.