Benchmark (level-)templated vs non-templated index functions

Currently, the indexing functions are templated with the corresponding refinement level.

But it is not clear, if that optimization is necessary (it is not even clear if the templates are better than non-templated indexing functions...). Therefore we should compare the performance impact of a non-templated indexing function.

If there is none, this would mean we can get rid of the templated indexing functions which would