Add assertions to code for testing invariants.

[jorisdral]

Originally suggested in #19 (comment).

We should add assertions for testing invariants (like positivity and normality) to the code in diff-containers. We can then enable these assertions in the test suite, which then warn us if definitions are breaking these invariants. Production code should disable these assertions, because they are probably expensive to compute.

[jorisdral]

With the rework done in #31, these invariants have disappeared. This issue can be closed