Better support for concolic runs with many iterations.

- Search strategy now runs multiple passes at increasing depth limits to pick up any new traces which were added by unexpected ("missed") executions.
- Added a new "overview" trace display (and options for --concolic-output-tree) which allows better high-level visualisation of really big trees.
- Added a script to automatically generate images and build an animation of the concolic execution process.
Merge branch 'master' of github.com:cs-au-dk/Artemis