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>
Added median_hilow

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

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

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

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

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

Signed-off-by: Matthias Kümmerer <matthias@matthias-k.org>
Merge pull request #348 from matthias-k/stat_summary

Stat summary
- [x] new geom
- [x] docstrings
- [x] tests