[product.l2] major refactor * all traits are now in namespace internal * added all variants of WeightedL2 * derived L2 from WeightedL2 * added over_integrate option
[product.l2] major refactor
* all traits are now in namespace internal
* added all variants of WeightedL2
* derived L2 from WeightedL2
* added over_integrate option

BarbaraV/dune-gdt,ftalbrecht/dune-gdt