Add code to autolayout petri net

* Uses graph stream spring layout
* Does not work yet
Provide hierarchical and organic layouts #5

* We use the jgraphx graph library to layout Petri nets for us it does
  this by:
  a) transforming a Petri net to a graph
  b) laying out the graph
  c) updating the Petri net components to their new layout
* Cannot yet handle arc points annotations etc, only places &
  transitions
Merge pull request #6 from sarahtattersall/feature_auto_layout

Feature auto layout