Jean-Baptiste-Camps/stemmatology: First CRAN release

First release to be published on CRAN. Lots of updates,

• Switched to igraph for network and 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 for all functions;
• rewrote code of some PCC.Exploratory functions to make them more efficient;
• various corrections and improvements;
• bug fixes.

See #43, #33 , #32.