Get rid of NLDI import

[ehinman]

Adding nldi module to __init__.py was a mistake: it causes dataretrieval to try to import the nldi module (which uses the optional dependency geopandas) every time dataretrieval or any of its modules are imported. This will throw an error if you try to install dataretrieval in an environment that does not also have geopandas. My bad. This PR fixes this issue. User can still use from dataretrieval import nldi or similar to use the module (if they also have geopandas).