Add tests that also perform de-trending steps.

[ekaterinailin]

What needs to be created or improved?

Testing the de-trending part is computationally intensive. We still need to test these parts of the code.

Goal:

Write tests that assert that de-trending is triggered when necessary, and leads to similar results each time.

• [x] Use mock_flc and test detrending on this one.
• [x] check if characterize_flares works as expected with the inject_before_detrending flag set.

What is the goal / expected behaviour?

I shall return helpful error messages when the attributes of a given FlareLightCurve do not have the de-trended part but need them, or catch these exceptions on the go if this is what we would expect as users.

[ekaterinailin]

You can close #16 when you close this one.

[ekaterinailin]

Now covered by sample_flare_recovery and its tests.