add a save function to NWIS

[mroberge]

Create a method for NWIS that allows user to save the dataframe as a local file. The current internal data structure could be saved as a parquet file, which does a great job compressing the data and working quickly.

Additionally, you could have the NWIS.get_data() method check for a file with the correct name before submitting a request to the NWIS. This would act as a cache, so that Jupyter notebooks that get re-run multiple times don't keep submitting the same request over and over.

[mroberge]

You could add a file = 'textname_here' parameter to the NWIS init method, and have the init method also submit the request to NWIS if the file doesn't already exist. If you wanted to overwrite an existing file, you could always submit a .get_data() request, and make sure that that method doesn't check for a file first.

[mroberge]

closed with #53