Update to spatial intro work in progress

This PR addresses the FIXME in the spatial-intro ipynb. Basically, I re-worded what is in the basemap docs:

> Calling a Basemap class instance with the arguments lon, lat will convert lon/lat (in degrees) to x/y map  projection coordinates (in meters). If optional keyword inverse is True (default is False), the inverse transformation from x/y to lon/lat is performed.

I don't know if that's how I would design the API. I'd probably prefer to have a specific class method that was in charge of unit conversion rather than using the class instance itself. I'm not in the spatial analysis field so perhaps there is a good reason for the choice. 

I also changed the class instance name from `map` to `bmap`. I think Randall changed it from my original `bm` naming convention (where I was just following the basemap example style), because he wanted to follow SWC's own advise of using descriptive variable names. I think that should be balanced with the python best-practice of not using variable names that conflict with python built-ins. `bmap` is a compromise, but I'm open to further revision.