Refactor registration of features in RTDC-Hierarchy class

[maxschloegel]

Due to the solution of #165, in a RTDC-Hierarchy instance the registration of features happens both in its apply_filter()-function and in its __getitem__()-function. Specifically this is the case for non-scalar features "image", "image_bg", "mask" (done in apply_filter()-function) and non-scalar temporary/plugin-features.

For clarity and cleaner code, this should be bundled into one external function being called in both, apply_filter() and __getitem__().

[paulmueller]

Other things that can be improved while fixing this:

• decorate the shape properties of the ChildBase subclasses with an additional functools.cache
• remove any key in dfn.FEATURES_NON_SCALAR from Accessing non-scalar, non-innate features in RTDC_Hierarchy objects leads to Error #165 (if it is still there)

The external function can be either

• a "require_feature" function that is called in __getitem__ for each feature or
• a "setup_features" function that is called always in __getitem__ and apply_filter

It should also be kept in mind that we have temporary and plugin features that, at any time point, may appear and disappear at the user's will. There should be tests against that to make sure that e.g. a hierarchy child does not return a temporary feature if that feature was deregistered.