unit tests for SDC

:exclamation: Please apply after PR #30 :exclamation: 

I changed the signature of `::echo_erro()` such that it not only prints the error but also returns it. This way we can easily assert its value.

There is one open point I'm having trouble with: How can I query the final time point such that I can call `AdvectionDiffusionSweeper::echo_error()` on the last node.

Contributes to fixing issue #25.