add snapshot tests

[mcol]

Right now only a few tests check the numerical results. As we are going to do more invasive changes, it's better to have a more precise way to check that we are not unexpectedly changing the results or the objects produced.

With testthat we can use snapshotting to record the expected output and compare it with that of the test runs.

[RLumSK]

Add first figure snapshot with 4ebe638