Fix gwlevels pcode

[ehinman]

This small PR adds a filtering step to the get_gwlevels function that ensures only the requested pcodes are returned. For whatever reason, the "new" gwlevels endpoint doesn't seem to recognize the parameterCd argument, even though it is still specified in the documentation: https://nwis.waterservices.usgs.gov/docs/groundwater-levels/groundwater-levels-details/

See Issue #179

[thodson-usgs]

Looks good to me. Normally, we should avoid patching the upstream API; however, this is a shorterm fix until the nwis services are deprecated.

[BojanMilinic]

The code itself looks good and all the checks at least pass on windows. I am interested in the "Did you mean to use assert instead of return?" error for the Ubuntu runs.

[ehinman]

The code itself looks good and all the checks at least pass on windows. I am interested in the "Did you mean to use assert instead of return?" error for the Ubuntu runs.

Thanks for identifying this bug with the services and taking a look, @BojanMilinic. @thodson-usgs and I plan to address this testing issue in a subsequent PR.