gnu: Add r-gridextra.

* gnu/packages/statistics.scm (r-gridextra): New variable.

gnu: Add r-gtable.

* gnu/packages/statistics.scm (r-gtable): New variable.

gnu: Add r-rmarkdown.

* gnu/packages/statistics.scm (r-rmarkdown): New variable.
* gnu/packages/haskell.scm: Do not select libxml2 from the xml module to
  prevent circular package module imports.

gnu: Add r-catools.

* gnu/packages/statistics.scm (r-catools): New variable.

gnu: Add r-bitops.

* gnu/packages/statistics.scm (r-bitops): New variable.

gnu: Add r-rcpparmadillo.

* gnu/packages/maths.scm (armadillo-for-rcpparmadillo): New variable.
* gnu/packages/statistics.scm (r-rcpparmadillo): New variable.

gnu: Add r-lattice.

* gnu/packages/statistics.scm (r-lattice): New variable.

gnu: Add r-gridbase.

* gnu/packages/statistics.scm (r-gridbase): New variable.

gnu: Add r-plotrix.

* gnu/packages/statistics.scm (r-plotrix): New variable.

gnu: Add r-readr.

* gnu/packages/statistics.scm (r-readr): New variable.

gnu: Rename r-data.table to r-data-table.

* gnu/packages/statistics.scm (r-data.table): Rename variable ...
(r-data-table): ... to this.
[description]: Clarify relation to data.frame.
[license]: Use correct license.

gnu: Add r-devtools.

* gnu/packages/statistics.scm (r-devtools): New variable.

gnu: Add r-rstudioapi.

* gnu/packages/statistics.scm (r-rstudioapi): New variable.

gnu: Add r-git2r.

* gnu/packages/statistics.scm (r-git2r): New variable.

gnu: Add r-httr.

* gnu/packages/statistics.scm (r-httr): New variable.

gnu: Add r-roxygen2.

* gnu/packages/statistics.scm (r-roxygen2): New variable.

gnu: Add r-brew.

* gnu/packages/statistics.scm (r-brew): New variable.

gnu: Add r-whisker.

* gnu/packages/statistics.scm (r-whisker): New variable.

gnu: Add r-rversions.

* gnu/packages/statistics.scm (r-rversions): New variable.

gnu: Add r-xml2.

* gnu/packages/statistics.scm (r-xml2): New variable.