Vectorize `terrain.py` methods

[rhugonnet]

This is planned for soon, using the convolutions added in #292 (moved to GeoUtils eventually, #300) Also, to echo a comment in #301, we should define the output dtypes of terrain attributes and use the default nodata of GeoUtils via `raster._default_ndv()'.

[erikmannerfelt]

What exactly do you mean with vectorize? Speed up? The np.vectorize decorator won't work as it's using numba, not numpy for the brunt of the work.