Move validation of class enum trait to run time instead of import time

[maxnoe]

Please describe the use case that requires this feature.

Currently, we define class traitlets like this:

class Foo(Component):
    bar_type = create_class_enum_trait(Bar...).tag(config=True)

this runs at import time of Foo. Which makes it impossible to use subclasses of Bar that are only imported at a later stage.

This is problematic for subclasses defined in other submodules than the base class and the containing component.

THis was triggered by the machine learning module, where ShowerProcessor has a class enum trait of Reconstructor but because ctapipe.ml.sklearn cannot import Reconstructor without also importing ShowerProcessor (as ctapipe.reco.__init__ imports it), the ml reconstructors are not yet defined when the class enum trait is created.

This is even more problematic for the case of plugins.

Describe the solution you'd like

Move the check if a provided string is an available class to the runtime (i.e. the validate method).

[StFroese]

So this is the reason for this:

ctapipe/ctapipe/reco/__init__.py

Lines 3 to 4 in 05beb25

	# reconstructors must be imported before ShowerProcessor, so
	# they are available there

[maxnoe]

Exactly