How to use a ProductStructureKernel with dimension-independent parameters?

[jayhop]

I want to use a ProductKernelwith the same base kernel but with different parameters.

As suggested suggested in #747 ProductStructureKernel is the way to go by setting ard_nums_dims on a base kernel.

My understanding is that if I set ard_nums_dims=D in the base kernel and num_dims=1 or num_dims=D in the ProductStructureKernel, the ProductStructureKernel will either do nothing or create a product of D base kernels each with D dimensions.