Structure improvements

• Allow different ploidy in outgroup (typically if there is only one outgroup sequence). Statistics requiring a consistent ploidy in the outgroup should be silently skipped.
  • All the architecture is designed to enforce a constant ploidy so allowing varying ploidy within the ingroup is likely to cause nasty side-effects for little benefit.
• Specify outgroup with struct_from_iterable (e.g. using a special markup instead of the population label).
• Ploidy option with struct_from_iterable.
• Add new function: struct_from_mapping
  • Signature: struct_from_mapping(indivs, ploidy=1[, pop[, clusters[, otg]]])
  • Example1: struct = struct_from_mapping(['A', 'B', 'C', 'D'], ploidy=2, {'pop1': ['A', 'B'], {'pop2': ['C', 'D']})
  • Example2: struct = struct_from_mapping('AABBCCDDE', {'pop1': ['A', 'B'], {'pop2': ['C', 'D']}, outgroup=['E'])