calculate average pairwise distance from distance matrix

[westram]

Provide a way to calculate average distance of species in matrix.

Motivation is to be able to compare

• -based average ingroup distance (AID; introduced with #653) with
• sequence-based distances as provided by distance matrix

Distance-sets to support:

• all species included in matrix (all possible pairs)
• only marked species (all possible pairs)
• all pairs of
  • selected species and not-selected species (in matrix)
  • marked species and not-marked species (in matrix)

Calculate (for each distance-set):

• arithmetic mean
• median

("selecting" one of the results could be used to set the limits in matrix view → allows to determine values above from values below the selected average)