Add config file parameters for tribe fission

[bmpotter]

This is just the 1st step in supporting splitting of tribes: add the config file parameters that control its behavior and the model structure for supporting more than 1 model of when/how to split.

• Add new parameters to config/Config struct and to mendel-defaults.ini:
  • popSizeToSplit
  • minGenToSplit
  • maxGenToSplit
  • SplitToNumPops
  • popSplitModel
• Add skeleton code for split model implementation in pop/models.go and pop/population.go
• Implement Population.ReadyToSplit():
  • returns total number of populations to split into (including itself), if:
  • current pop size >= popSizeToSplit and current gen >= minGenToSplit
  • or current gen >= maxGenToSplit