Helmholtz operators are missing initializers

[astanziola]

From https://discordapp.com/channels/1116725189972078639/1151518909254610984 by Wael - MIT:

I successfully ran the initial value problem you show in the documentation where jwave.png is loaded and uses as sources. However, I've been running into issues trying to recreate the results from the helmholtz problem test. The specific issue comes from this line: params = helmholtz.default_params(src, medium, omega=omega), which is always returning None for params. Would you mind helping with this? I see that default_params uses the method _bound_init_params from jaxdf.core, but for some reason the params passed back are always None. Thank you!