Add some benchmarks

Three layers :
- dxl package
- controller
- robot synchronization
Cosmetic correction in the benchmarks.
Add notebook for plotting benchmarks.
Update plot notebooks.
Remove the V1 vs V2 cmp.
Merge pull request #34 from poppy-project/benchmarks

Add benchmarks tools
Merge branch 'master' into vrep
Adapt benchmarks to the new controller API.