Optional: Add adhesion gripper

[SimpleGeometry]

An adhesion gripper should be possible now after MJX 3.2.6 with the equality active bugfix. I believe you can do it by making an equality in the XML file between the EEF and the cube; conditionally turn this on or off depending on how close the EEF is to the cube (you can either make it on contact, i.e. really close, or a bit away, like a magnet/air suction). If you want the attraction to be slower you can probably fiddle with solimp and solref.