changed .check_against_xsd() method so that certain nodes can be ignored in the validation of the xml file

resolved #12 (renaming to categorisation)
Updated documentation accordingly
Checked and updated all example problems

Fix in sink(s) visualisation

Implemented Metis graph partitioning algorithm (note that this requires installation of Metis on the system).
Created graph partitioning method and graph visualization.

Added analysis of XML/XSD knowledge base for coupled math problem.

Added simple adjacency matrix plot function

Solved issue with floating nodes in graph.
Added more advanced graph plotting options (color_settings) for sinks and node types.
Created separate sellarProblem.py file (to be used as test file during development).

Added input checks / knowledge base checks.
Added FPG method based on multiple sinks.

Added grouping of nodes according to level in XML Schema.
Added creation and node analyses of the MCG.
Added creation of FPG according to single sink.