Create sanity checks

[patnr]

There should be tests that make sure that the results of the algorithms are sensible.
These don't necessarily need to be part of automated CI tests,
as mentioned in #40, but rather the examples.

I suggest to use a trivial case, for example, for DA:
5d, no dynamics, observe (directly) 1 component.

• posterior mean/cov should nearly match theoretical.
• test all of the methods
• test for different configs (ensemble size, iteration numbers, with and without localisation)

For optimisation, for example: a 5d quadratic objective

• Check optimum is found
• Check number of iterations not excessive.