Add tests for single unit operation

[schmoelder]

A but was discovered in #28, where the solution was not returned correctly if the simulation only contained a single unit operation. This was fixed but unfortunately, we currently don't have tests for single-unit-operation systems (because currently we mainly use the LWE function).

We should improve this in the future.

@ronald-jaepel since you reported the issue, could you please provide a MRE?

[schmoelder]

After discussion with @ronald-jaepel , we decided to postpone this issue a bit. We should re-evaluate whether we want to continue using createLWE from CADET-Core to parametrize our tests or whether it makes sense to migrate some of the functionality out of C++ to improve the usability / flexibility. See also #47