Get USGS Salinity data

[SorooshMani-NOAA]

It seems the same NWIS API provides the salinity data as well. We just need to make sure the wrapper supports getting that data.

Currently only water and current data are extracted:

searvey/searvey/usgs.py

Lines 54 to 63 in abf5851

	USGS_OUTPUT_OF_INTEREST = (
	"elevation", # Overlaps some of the specific codes below
	"flow rate", # Overlaps some of the specific codes below
	"00065", # Gage height, feet
	"62614", # Lake or reservoir water surface elevation above NGVD 1929, feet
	"62615", # Lake or reservoir water surface elevation above NAVD 1988, feet
	"62620", # Estuary or ocean water surface elevation above NAVD 1988, feet
	"63158", # Stream water level elevation above NGVD 1929, in feet
	"63160", # Stream water level elevation above NAVD 1988, in feet
	)