Fix pcode endpoint

[sblack-usu]

The pcode endpoint does not work because the tool provides query parameters as a dictionary, which reduces the multiple "show" key query parameters to only the last one provided. See code here

https://github.com/UCHIC/dataretrieval/blob/master/dataretrieval/nwis.py#L367

This will also have to be fixed in the query method here

dataretrieval/dataretrieval/nwis.py

Line 163 in 224515c

def query(url, **kwargs):

Modifying the query function may have a cascading effect causing a lot of refactoring... I think it's worth exploring, but considering there are few tests, we may want to establish tests around the library before doing such work.

Acceptance Criteria:

• Fix the pmcode method to use a list of tuples for the query params,
• Either refactor the query method to support list of tuples or create a new issue describing the work necessary.