This includes a utility function to aid IO with bands based on data
type, and a wrapper class for working with in-memory GDAL raster
datasets and bands for use with GDAL functions in cython.
Includes additional validation logic for data type in rasterize.
Includes additional test cases for data type support.
Remove test data from sdists. Check for it when running tests and
direct users to download location. No automated download... I don't
expect many users to be running the tests from an sdist.
Closes#133.