Igraph

[Jean-Baptiste-Camps]

• Switched to igraph for stemma plotting ;
• enhanced visualisations:
  • stemmata plotted as tree
  • dotted lines for uncertainty;
  • created special layout where height in the stemma is proportional to the number of variants from the model;
  • added color, titles, etc.
• profiled the PCC.Stemma functions to gain efficiency (exec time /2);
• simplified PCC.Stemma outputs and documented it;
• added tests;
• various corrections and improvements;

Partially resolves #31 . Progress on #18 , #26