add additional properties to species and chromosomes

[petrelharp]

Species:

• sexual or hermaphroditic? (add "sex determination" #1146)
• recombination in which sexes? (Drosophila recomb rates should be divided by two! #1743)

Chromosome:

• chromosome type

Other things?

Also note #383, where there's discussion on how to handle sex chromosomes with msprime.