Fix for DiagonalMatrix * OtherMatrix

Summary

This MR provides an overload for the matrix-matrix product involving a DiagonalMatrix as first factor. This was not working properly before, if the second factor is a transposed view of a matrix, see #400 (closed).

In addition to the operator* overload for DiagonalMatrix and some other dense matrix, this MR implements the operator[] overload for the TransposedMatrixWrapper in order to implement the matrix-matrix product. If someone has a better idea, please comment.

• Needs documentation of the new methods.
• Needs some additional tests of the methods.
• Needs !1496 (merged)

Closes #400 (closed)