Merge pull request #88 from ipa-fxm/hydro_dev

fix xacro:include tag
New stl files for cob4
merge
Merge error
deleted offset
Merge pull request #89 from ipa-nhg/hydro_dev

Stl files for cob4 and defined colors
remove material physic properties of wheels to use default, fixes #90
Merge branch 'hydro_vel_control' into prace_changes