Fix addon filter from entry_points

I just noticed that the LitPixelsCounter addon was not present in the schema of AgipdCorrection. This is the only addon using the entry point extra functionality to specify detector comptabibility - from setup.py:

"LitPixelCounter = calng.correction_addons.litpixel_counter:LitPixelCounter [agipd]"

And I guess entry points' extras attribute used to be nested. At some point, this must have changed - maybe because I in response to some deprecation warning updated the entry point listing to:

entry_points().select().get("calng.correction_addon", [])

The fix is trivial, of course. I included a small test to see that add_addon_nodes at least manages to figure out this much. And that test told me I should remember add_addon_nodes for two detectors where it has never been used - debatable whether the existing addons should really be available for Gotthard2Correction, but the mechanism should at least be there.