Feature/scalar tests

Added a test for the scalar example, that verifies that it reproduces (approximately) the theoretically expected convergence order.
Also modified the scalar example, so that it fits into the testing framework.
So far, only up to six Lobatto nodes are tested.

Also, the MATCHER used in EXPECT_THAT is a pretty ugly construction and one can probably using something that is much more clever.
`std::vector::size()` returns `size_t`. We should use that as well and don't implicitly cast it to `int`.