copyWindowStructure should not set the matrix build mode to 'row_wise'

Rather, it should take the value from the matrix that is being copied from.

In particular, BCRSMatrix::copyWindowStructure is being called as part of BCRSMatrix::operator=. Previously, that method set the matrix build mode to 'row_wise' unconditionally. This does not pose problems until the matrix is asked to be resized later, because that only works for matrices with the 'random' build mode.

mentioned in issue #73 (closed)

mentioned in merge request !300 (merged)

After a discussion with @smuething we both decided to simply give up on this. The BCRSMatrix internals are so convoluted that it seems impossible to find a simple fix for this.

closed

mentioned in commit 4396b671