Added 'invert' parameter to make_transition.
The new parameter passes straight through to gdal_rasterize's 'i' parameter, indicating that it should return the inverse of the raster it ordinarily would. This is useful if the polygon passed to the function represents a landmass rather than a body of water. For example, if a user downloads a polygon from GADM representing the administrative boundary of a coastal state, province, or country, and then passes that polygon to make_transition, glatos will assume the polygon is a body of water, which is the opposite of what we want. With this parameter, we can simply invert the resulting raster, which will give us an object that treats land as land and water as water.