Load a GA SRTM DEM ESRI grid into a numpy array

Uses GDAL to load the grid file into a numpy array. Calculates the
lat/lons for the data coverage.

Includes a skeleton context manager for loading GDAL files using 'with'.
Doesn't yet do any error handling etc, or even close the dataset (the
API seems to be lacking a close?).

Requires a SRTM DEM (digital elevation model) data grid from Geoscience
Australia. This data is Creative Commons, and can be acquired from
http://nedf.ga.gov.au/geoportal/catalog/main/home.page