Population generator

These models derive species viewing hotspots from entry data.  This information will be displayed on the species show page.  Users can see where there have been the most sightings of a species.

Doings so requires that I save a population.  Populations have many entries. They are associated with a species, latitude, and longitude.   

* EntryObserver send the entry info to an EntryPopulationHandler.
* Handler tries to find a nearby population using PopulationFinder. 
* If it doesn't find a nearby population, it creates populations for nearby entries using PopulationSaver.
* Then it creates an association between the entry and the nearest population., with PopulationMembershipSaver.
* Each of these models relies on other models which find groups of nearby entries, and the geographic centers of them.  
Unused method argument - `cluster`. If it's necessary, use `_` or `_cluster` as an argument name to indicate that it won't be used. You can also write as `check_cluster_quality(*)` if you want the method to accept any arguments but don't care about them.