resolves #16 (creation of XDSM with MDF and Gauss-Seidel)

Added XDSM_writer package from Lambe and Martins to KADMOS folder
Started on get_PSG_for_MDF
Started on create_XDSM (for MDF)

Started function to get the PSG for the MDF method, updated documentation, updated graph plotting for PSG process flow.

resolved #4 (added a search function find_all_nodes that searches a node of a graph on category, subcategory, and attribute values)

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.