Add NLDI data retrieval functionalities.

[pkdash]

Here are some examples of function to retrieve NDLI data:

• nldi.get_basin() - returns a geodataframe with a polygon geometry
• nldi.get_flowlines() - returns a geodataframe with a polyline geometry
• nldi.get_XYZ() - returns a geodataframe with a point geometry
• nldi.search() - returns a JSON object

[thodson-usgs]

Excellent, let's call XYZ nldi.get_location or get_point. I should've been more careful in my specification, but I meant "XYZ" figuratively as in "to list off several examples".

There are several point datasets within NLDI. I think a single get_point function is fine, but you might choose to split these further into "get_x", get_y", "get_z", etc. :)

Thanks for the update.