Dataretrieval

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* Removed deprecated functions.
* Added siteInfo and variableInfo dataframe attributes to all data retrival functions
* Improved help files
* Added data file (pCodeToName) that shows the association of USGS parameter code to characteristicName

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* Overhaul of function names based on initial user feedback
* Metadata attributes added to returned dataframes
* EGRET specific functions moved to EGRET

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* Changed naming convention:
* Changed WaterML2 rbind fill from plyr function to dplyr. Removed plyr import, added dplyr.
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* Updated getNWISSiteInfo to retrieve multiple site file datasets at once using a vector of siteNumbers as input argument.
* Updated error-handling for Web service calls. More information is returned when errors happen
* Added some basic processing to Water Quality Portal raw data retrievals. Date columns are returned as Date objects, value columns are numeric, and a column is created from the date/time/timezone columns that is POSIXct.
* Added very generalized NWIS and WQP retrieval functions (getNWISData, getNWISSites, getGeneralWQPData, and whatWQPsites) which allow the user to use any argument available on the Web service platform.

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* Deprecated getQWData, updated readWQPdata to take either parameter code or characteristic name.
* Changed the name of raw data retrievals to: readNWISqw, getNWISunitData, getNWISdvData, and getWQPqwData (from: readNWISqw, retrieveUnitNWISData, retrieveNWISData, getRawQWData)
* Added NA warning to getDVData function
* Updated mergeReport to allow for Sample data with different measurements taken on the same day

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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).