[WIP] Change vector density when zooming

This change sets a number of vectors to show when at minimum zoom, and quadruples the number at each increased zoom level, as long as there are enough vectors available.  In practice, this doesn't span the entire zoom range, so at higher levels the density will decrease.

Performance is an issue at higher zooms.  Perhaps something can be done to avoid spending time updating vectors outside the frame?

Closes #7.
After fiddling around with this for a bit I think the best way to go is keep a separate array of all the vector layer objects and slice it at `displayPoints`. We can then iterate over the first half and add them to the map and iterate over the second half and remove them. Leaflet apparently takes care of the cases don't make sense (removing when not present or adding twice). After that's done, the rest of the redrawing code and everything else in the app should work unmodified.