make access to off diagonal entries of DiagonalMatrix return zero

There is probably some other idea here that I'm missing but having the DiagonalMatrix return the diagonal value for all entries in a row makes generic programming difficult. Sometimes using mv etc is not an option and I needed access to jacobian[i][j]. Is the idea then to always copy jacobian into a DenseMatrix, which in almost all cases will be an overhead except if jacobian is a DiagonalMatrix or is there some other concept I'm missing? PS: perhaps bugfix should be feature...