Improvements to mapping cells.

@gvwilson my contributions toward https://github.com/swcarpentry/bc/pull/387.

I've made a few new cells just about the basemap figure, no points yet.

Then an explanation of the coordinate system and details of transforming from lat, lon to a map coordinate system. Plus what I think is a decent explanation for why bmap takes lon, lat args.

After which, a couple cells about plotting points on the map.