Make filledArray() C++11-compatible

See #99 (closed).

@martin.nolte, @andreas.dedner: This is mostly a 2.6 issue, if I understand @markus.blatt correctly in #99 (closed). My plan is to simply remove constexpr for 2.6, but I'd like to keep it for master. Can I

1. make a patch directly for the release-branch, or
2. do I have to go through master, and basically revert it there later once it's been cherry-picked to 2.6?

mentioned in commit 6a8d6305

mentioned in merge request !412 (merged)

!412 (merged) fixes this and has been merged.

closed