Bugfixes: initialize warnings

Signed-off-by: Matthias Kümmerer <matthias@matthias-k.org>
New stat: stat_summary, only with mean_cl_boot right now

Signed-off-by: Matthias Kümmerer <matthias@matthias-k.org>
stat_summary: implemented mean_sdl, implemented fun.y etc, allow own functions

Signed-off-by: Matthias Kümmerer <matthias@matthias-k.org>