Dataretrieval

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* Reworked whatNWISdata to allow the full flexibility of `...`
* Check for dates in instantaneous data, allowing requests for recent data to be routed directly to waterservices.gov.

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* Converted vignette to html

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* Added National Groundwater Monitoring Network services
* Converted WaterML2 code from XML to XML2 dependency. NOTE: The output of this the parseWaterML2 function has changed. This is a first step to a more robust WaterML2 parser
* Added new Water Quality Portal features: whatWQPmetrics, whatWQPdata, whatWQPsamples
* Bug fix on converting GET -> POST
* Bug fix on state abbrieviations
* Minor updates to internal code to improve efficiencies.

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* All USGS web services are being flipped from http to https. This version of dataRetrieval will attempt to access https, if that fails, will attempt http.

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* Response to readr 1.0.0 updates
* Added NWIS Water Use data
* Added NWIS Statistical Service data
* Added message on updates to NWIS data on startup message
* Automatically retries web service calls when status code is greater than 400
* More integrated use of pkg.env to set base URLs

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* HEAD requests can be returned in readWQP* functions. This can give some indication on amount of data available.