Name/lookup functions for Species_skill.
Change all references to "Species" to "Race" for consistency.
Improve Race_datum::Race_datum().
Add a race to City.
In City::City(), init race to RACE_HUMAN.  Use our race's properties to alter logging, farming, mining, birth rate, citizen birth ratio, low/high tax rates, and food consumption.  Support infinite wood sources, although this should never happen.