add approxsphere to MARStypes
add joints to groups using correct joint names
only add non-empty groups
use lists for chains, not space-separated strings
implement srdf-export of chains, links, coll.spheres
format and document srdf export
add operator for defining chains
add safeguards for group derivation

Objects which are not of type 'link' are now ignored.
Especially, the automatically created 'RigidBodyWorld' group is ignored.
set mass op now uses old mass or rigid body mass
implement setCollisionGroup operator
implement collision bitmasks for srdf

The robot dictionary now includes collision bitmasks for collision
objects and combined (bitwise |) collision bitmasks for links.
If links don't overlap at all (bitwise & yields 0), an SRDF tag is
added to disable the collision between those two links.