MatrixObj: add "minimal" MatrixObj and VectorObj implementations

[fingolfin]

... which implement exactly the required operations and nothing more. The idea being that we can then use these together with the MatrixObj / VectorObj test suites to test all our generic implementations.

To prevent users from accidentally using them we maybe shouldn't make them part of the library but instead load them from the test suite only?

(@wucas wanted to work on this, and perhaps already did, but I am filing an issue so this isn't forgotten)