Improve/xml deserialization

This merge request adds the customizing pattern from the json serialization to the xml serialization - which is now not necessary anymore for it's original usecase, but will be useful for parsing stripped xml elements in the http api.
Furthermore, the functions read_aas_xml_file_into() and read_aas_xml_element() were added. read_aas_xml_element() should originally accept a type and a decoder class, but because mypy doesn't narrow down type variables for if type_ is SomeType and the issue was still present with if issubclass(type_, SomeType), despite it narrowing down the type, I decided to change the API and just pass a constructor function to it, as I think that it is prettier than telling mypy to ignore the type at the end of each line.

see #63 (closed) #91
fixes #74 (closed)

changed the description

mentioned in merge request !58 (merged)

@leon.moeller: How is your progress on the issues above? I'd like to get this branch merged, especially for the minor change for Qualifier objects.

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• 6b64c17a - make failsafe and stripped class variables

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• f05fd35a - raise ValueError in read_aas_xml_element(), if none of the previous cases match

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• 3f358cfb - adjust error message example in module docstring

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• 6476e8f9 - adapter.xml.xml_deserialization: make failsafe and stripped class variables
• da71c0bb - adapter.xml.xml_deserialization: raise ValueError in read_aas_xml_element(),...
• ca847eaf - adapter.xml.xml_deserialization: adjust error message example in module docstring

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• 447438dc - adapter.xml.xml_deserialization: change type_ to Referable

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Recent changes LGTM.

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• 395d6593 - model: change key type of KEY_ELEMENTS_CLASSES to Type[Referable]

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