This commit migrates the codebase to use pysheds instead of TauDEM and ArcPy. Key points include: * Package is now fully open source * All dependencies are Python packages - conda install no longer necessary * The `dem` and `stream` modules (using TauDEM and ArcPy) have been replaced by the `watershed` module (using pysheds) * Raster backends have been consolidated into the Raster class Note that this tag intentionally breaks the `segments` and `models` modules, which will be refactored over the next 1-2 tags.