Release 0.3.1

Hello, folks! This is a time for a minor release of GOLEM coming with several new features and some incremental improvements.

Highlights are:

• Added several crossovers that exchange edges between graphs, useful for Bayesian network evolution (#58)
• Added Sensitivity Analysis for graphs that check how important certain nodes and edges are for the fitness of the graph (#62)
• Graph visualisations now include label viz (used in particular in Sensitiviy Analysis) #76
• Add new example that implements Tree evolution using fast Edit Distance metric (#60), check it out in tree-search.py
  

Fixes:

• #67
• #75
• #84
• Usability fixes from #86