Basin parallelization

Overview

This merge request adds the _utils.basins module, which adds parallelization options for building terminal basin rasters. The merge also updates the Segments class to support this module, and removes code for the unused nested basin rasters.

Notes for reviewer

1. The key new addition is the _utils.basins raster - all parallelization functionality is located there.

2. The diffs for segments.py will look large due to some cleaning/reorganization. The important changes are as follows:

   • Added the locate_basins method
   • Simplified the _update_basins method,
   • updated docstrings for the raster, save, and geojson methods

Closes #62 (closed)