Plumbing for active_collision_options.collision_groups
Add test for active collision options

Test collisions for only specific bodies/groups
Fix handling of default constructed `Body` objects.
Handle the active_collision_options.group properly

Now only elements in the groups listed will be checked for collisions.
Pass collision group names from MATLAB to mex
Fix constructModelmex for collision elem. w/o grps
Merge pull request #241 from avalenzu/active-collision-options

Active collision options