Fixes #4138: Add support for loading Gephi GEXF file format

[vga91]

Fixes #4138

• Added procedures for reading GEXF files.
• Copied and pasted code from APOC Core and put it into XmlUtil.java
• Changed parseXML method to be able to handle properties of gexf files. To handle the name of relations we parse the kind tag. While for node properties we change from doing the parse of the data tag to the attvalue tag. Also, to parse the label in the node tag you parse labels instead of labels as compared to how it is done for graphml.
• Multiple label support for nodes. Example <node id=“0” label=“:Gephi:Webatlas”>.
• Types tested: string, integer, long, foat, double, boolean, liststring, listboolean
• Improved ExtendedUtil.convertValue with toLowerCase(), to be used by both Parquet/Arrow and Gefx formats
• Improved ExtendedTestUtil
  • added arrays assertion to assertMapEquals
  • added assertRelationship

[RobertoSannino]

nodes and relationships OR vertices and edges

[RobertoSannino]

add also documentation example for malformed GEXF files