Dataretrieval

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* Package httr is now used for web service retrievals

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* Package readr now used for tab delimited parsing
* readr functions used to determine column types. Mostly, this produces the same results.
* In the case where there is text in a numeric column (specified by the RDB header), these now remain characters (previously was converted to numeric)
* Columns that come back from web services as integers remain integers (previously was converted to numeric)
* Added reported time zone code information. dateTime columns by default get converted to UTC, but the original time zone code (tz_cd for instance) is appended to the data frame.

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* Converted all Water Quality Portal queries to sorted=no to greatly improve retrieval times
* Converted gwlevels in readNWISdata to rdb

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* Changed the output of tz_cd (in all *NWIS functions) to the timezone that is reported in the data frame. Before this version, it was the time zone reported by NWIS. This was confusing however since the dateTime columns default to UTC.
* Minor bug fixes
* Help file updates
* Improved query options in readNWISdata and readWQPdata functions.

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* Expanded readNWISdata to qw data
* Improved citation file

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* Removed tests that call external web services, server upgrades have been causing false negative results
* Added functionality to readWQPdata to allow R objects as arguments rather than just character strings
* Added parameterCd='all' option for qw calls.
* Exposed sysdata constants