Add missing `#include <numeric>` for `std::accumulate` (!21) · Merge requests · staging / dune-functions

This fixes a build failure with GCC 6.

Thanks to Logan Rosen <logan@ubuntu.com> for the patch.

Bug-Debian: https://bugs.debian.org/823550

cb97e30a

Add missing `#include <numeric>` for `std::accumulate`