Dataretrieval

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* Added getNWISDataAvailability function to find measured parameters and period of record information for a requested station.

* Added constructNWISURL function to get the URL that is used to retrieve the data.

* Added getSampleSTORET function to get STORET data directly in Sample dataframe form.

* Fixed a small leap year bug by changing day of year by making Feb. 29 always 59, and March 1st always 60 (even in non-leap years).

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* Improved documentation, especially example functions.

* Improved vignette for a more complete walk-through.

* Expanded the capabilities to retrieve raw data from the web services.

* Added Storet data retrievals in readWQPdata function