Problem with DirectionalTargetDerivative

If I use the kernel created by `DirectionalTargetDerivative(HelmholtzKernel(2))`, in pytential the following error ensues when I try to create the operator `sym.D(kernel,...)`

[dir_tgt_derivative.log](https://github.com/inducer/sumpy/files/2851827/dir_tgt_derivative.log)

Start by reproducing the reported failure with DirectionalTargetDerivative(HelmholtzKernel(2)) and sym.D in pytential, then inspect the attached dir_tgt_derivative.log for the failing operation. Trace the corresponding Sumpy kernel and operator-generation entry points; done means the operator can be created without the reported error.

Vom Indexierungsmodell aus dem Issue-Text verfasst.