Add support for StreamStat about pynhd HOT 3 CLOSED

@cheginit There is a specific workflow in mind for the StreamStats services and that is to essentially compute estimates for streamflow characteristics at ungaged locations using USGS regression equations. In StreamStats the data and regressions are typically state specific, but also are sometimes region specific. So, while a lat/lon should tell us which state and which data to use, there are times when you could be using 2 or 3 different datasets. The StreamStats services are definitely not as clean as the NLDI services, but should result in better delineations. Depending on the purpose, that may or may not be important.

I think you're already looking at the documentation of the services, but if not here is the link: https://streamstats.usgs.gov/docs/streamstatsservices/#/
rcode is the 2-3 digit code that identifies the state or regional boundary. Most of the time it is state boundaries, but in some cases it is regional boundaries (i.e. Delaware River basin). StreamStats also uses a stream grid representation to show users where to click. In most cases that stream grid should line up with 1:24,000 NHD flowlines whereas NLDI is based on 1:100,000 flowlines. To get the stream grids for each state (to know where to click) you can find them here: https://streamstatsags.cr.usgs.gov/StreamGrids/directoryBrowsing.asp

Finally, I just wanted to note that I'm not necessarily requesting you implement StreamStats services or delineation, I was just letting you know the differences. If you do start using StreamStats services, please let us know. We eventually plan to move to a user key system so when we make updates we can notify our users. Right now we don't know who has implemented the services in their own application.
Thanks!

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@USGSPMM3, Thanks for the detailed explanation. It makes sense.
I was wondering how these estimations are different from this item from ScienceBase which includes over 30 catchment-scale characteristics including streamflow's over CONUS. PyNHD's nhdplus_attrs function pulls these characteristics from the item.

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Considering that streamstats already exists, there's no point in adding this.

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