add helper function, might be nice for parametrized tests

[dneise]

just a tiny helper function breakout with a test.

I thought it might be nice to have parametrized tests (which I just learned about :-) to include any new subclass automatically.

so go from something like this:

ctapipe/ctapipe/image/tests/test_image_cleaner_component.py

Lines 9 to 11 in 384b641

	@pytest.mark.parametrize(
	"method", ["TailcutsImageCleaner", "MARSImageCleaner", "FACTImageCleaner"]
	)

to this:

@pytest.mark.parametrize(
    "method", ImageCleaner.non_abstract_subclasses().keys()
)

In addition, I thought it might be nice to allow people to learn about the available subclasses in a python session. Unless there is already a way, which I forgot about ...

[codecov]

Codecov Report

Merging #1203 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1203      +/-   ##
==========================================
+ Coverage   86.15%   86.15%   +<.01%     
==========================================
  Files         182      182              
  Lines       11546    11550       +4     
==========================================
+ Hits         9947     9951       +4     
  Misses       1599     1599

Impacted Files	Coverage Δ
ctapipe/core/tests/test_component.py	97.52% <100%> (+0.02%)	⬆️
ctapipe/core/component.py	95.65% <100%> (+0.19%)	⬆️

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