Feature/xml serialization merge

See merge request acplt/pyaas!11

# Conflicts:
#   test/adapter/json/test_json_serialization.py

* qualifier_to_xml
* value_reference_pair_to_xml (and _iec_value_reference_pair_to_xml)
* property_to_xml
* range_to_xml

The **kwargs are passed to the tree.write() function of lxml

The changes were made in the following functions:
* qualifier_to_xml
* value_reference_pair_to_xml (and _iec_value_reference_pair_to_xml)
* property_to_xml
* range_to_xml

model.File.value is Optional[PathType] (string), but it wasn't tested
for None before. Now if File.value = None, it is not serialized

ValueDataType occurs and is fixed with model.datatypes.xsd_repr() in:
* qualifier_to_xml
* value_reference_pair_to_xml
* property_to_xml
* range_to_xml

add _iec_lang_string_set_to_xml
add _iec_reference_to_xml
add _iec_value_reference_pair_to_xml
add _iec_value_list_to_xml
fix some minor serialization mistakes

See Issue #57

See Issue #56 for detailed explanation

Feature/compliance tool

See merge request acplt/pyaas!10

adapter.xml: use lxml.etree.Element nsmap parameter instead xml.etree.ElementTree.register_namespace in serialization

examples: Add shebang, executable-bit and absolute imports

See merge request acplt/pyaas!15