Jean-Baptiste-Camps/stemmatology: v0.2.3 ``Namur''

Minor release, with many improvements:

• added tests for the PCC.Stemma functions;
• rewrote part of the code, to make it less verbose and more efficient;
• corrected some minor bugs;
• improved documentation;
• added ask=TRUE and verbose=FALSE to the PCC.Stemma functions.
• 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.