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)