Rewrite the xml parser in vpr to be based on DTD and auto generated.

Currently the xml parser in vpr is hand coded.

It would be good to rip it out and replace it with a auto-generated xml parser based around a DTD which defines the file format.

Create a XML definition using DTD, XSD or Relax-NG -- unsure the best format... See;
- https://www.tutorialspoint.com/xsd/index.htm
- http://www.relaxng.org/tutorial-20011203.html

Existing XML file formats in order of importance;
* "rr_graph.xml" format - http://vtr-verilog-to-routing.readthedocs.io/en/latest/vpr/file_formats.html#routing-resource-graph-file-format-xml
* "Packed netlist" format - http://vtr-verilog-to-routing.readthedocs.io/en/latest/vpr/file_formats.html#packed-netlist-format-net
* "FPGA architecture description" - http://vtr-verilog-to-routing.readthedocs.io/en/latest/arch/index.html

New XML file formats needing to be created;
* placement file format - http://vtr-verilog-to-routing.readthedocs.io/en/latest/vpr/file_formats.html#placement-file-format-place
* route file file format - http://vtr-verilog-to-routing.readthedocs.io/en/latest/vpr/file_formats.html#routing-file-format-route

The idea is eventually to auto-generate parser / code structures from the definitions; IE see --
- http://www.davekuhlman.org/generateDS.html
- https://www.liquid-technologies.com/xml-schema-to-cpp
- https://www.codalogic.com/lmx/?xmlfwd-xml2cppcom
- https://www.codesynthesis.com/products/xsd/
- https://www.genivia.com/doc/databinding/html/index.html