Add numba acceleration and major refactor

[mdbartos]

• Hydrologic processing functions now accelerated using numba.
• Hydrologic functions now only accept Raster objects as inputs.
• Datasets as named parameters of Grid instance have been removed.
• Spatial referencing features are now more fully featured and consistent.
• Added new hydrologic processing functions, including stream_order and distance_to_ridge.
• D-infinity routing support added to many new hydrologic processing functions.
• flow_distance has been renamed to distance_to_outlet
• resolve_flats now much more successful at removing flats.
• All ViewFinders are now regular, and defined by affine, shape, crs, mask and nodata.
• cell_areas function removed, because all grids are now regular.