[Release][Array] Provide a function for creating an array filled with copies of a single element

The new YaspGrid constructors require an array with the number of cells
in each direction. Unfortunately, there is no array constructor that
takes a single element and copies it to all array elements.

This patch adds a simple function Dune::fill_array() to do just that.
(cherry picked from commit 2fff527599173a8da0ce38a7c9d7f38053f04122)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>
[Bugfix][array] Remove left-over debug output

Thanks, Oliver!
(cherry picked from commit 80f3adbe56ee43f83474cc3be4ede1fa5121cd0a)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>