Seppl

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- reworked excluding of classes

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- `_determine_from_entry_points` method of `ClassListerRegistry` class now checks whether
there the attributes tuple has any elements (i.e., whether the optional `:function_name`
was provided)
- message `X records processed in total` now only output at the end

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- `ClassListerRegistry` now safely removes any excluded class listers before locating the classes

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- `ClassListerRegistry` now removes any excluded class listers before locating the classes

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- the `execute` method no longer counts `None` items returned by the reader
- added the `seppl.ClassListerRegistry` class that offers a more convenient way of
discovering classes via a function that returns a dictionary of superclasses and
the associated modules to inspect; with this approach only a single entry_point
has to be defined in `setup.py`, pointing to the class lister module/function

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- added the dummy type `AnyData` which is used by default in the `check_compatibility`
method for a *match all* (ie can be used for general purpose plugins)