Fix/add/test Mat bindings

[alxbilger]

Mat bindings were malfunctioning and not even tested.

Changes:

• add binding for trace
• fix in-place assignment with modification (*= and /=). It did not work, although the bindings were defined, because the C++ operators returns void instead of a reference to itself, which is the norm.
• add support of matrix product
• add support of VecNoInit type in bindings of Vec. Without that, it's not possible to do anything with mat[i] (because mat[i] is of type VecNoInit and not Vec).
• add some unit tests