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.