UPDATED FUNCTIONS:
 * Bk_plot - now adds dots for the asymptotic lines in case of NA's
   * Bk - now calculates cutree once for all relevant k's - and only then goes forth with FM_index.

BUG FIXES:
   * FM_index_R - now returns NA when comparing NA vectors (when, for example, there is no possible split for some k), instead of crashing (as it did before).
   * Bk_plot - now won't turn one dendrogram into hclust, while leaving the other a dendrogram.