Mention R-Snippet general requirements

[jpfeuffer]

When running the Tutorial on a fresh R, I stumbled across some difficulties in the
Metabo workflow.
The general R requirement (install.packages('Rserve')) for using a local R Server was missing. KNIME tells you this but maybe we could mention that beforehand.
The same holds for install.packages('Cairo') for bitmap handling (at least on MacOS).

[jpfeuffer]

Also I could not install devtools and ggbiplot from within the R Snippets (needed for the PCAbiplot, rest worked fine). We should either remove that node or really make a section about R dependencies (which explains, which packages to install beforehand).

[cbielow]

You could create an R package .. dependencies will be automatically installed via the definitions in the package DESCRIPTION file.
If one needs devtools anyway, install_github() comes with it, so installation boils down to a single command.

If that's too much then:

if (!require("packageXY")) install.packages("packageXY"); require("packageXY");
should work at the beginning for each script.

(not sure if that helps in any way--- not really familiar with the R part of the tutorials).

[mwalzer]

so we had this before the 2015 UM: #8