Fixes for the object_comparison() method.
Added a missing import.
Created the SpinList.from_xml() method.
Created the ResidueList.from_xml() method.
Created the MoleculeList.from_xml() method.
Added a test to xml_read_element() to check if the pipe is empty.
Created the RelaxFromXMLNotEmptyError class.
Changed the call from MoleculeList.xml_read_element() to MoleculeList.from_xml().
Wrote the PipeContainer.xml_read_element() method.
Wrote node_value_to_python() to convert the XML nodeValue to a python object.