Update list of required R packages

Package "logging" is required.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
Make sloccount and understand analysis optional

Since understand is a proprietary tool, not every codeface
user will have it installed. Allow for making the respective
analysis pass optional. For reasons of symmetry, also make
sloccount optional, but default to doing the pass.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>