urdf: removed common.xacro

This was just a copy of uos_common_urdf/common.xacro. We copied that
here because we don't want to depend on another stack just for that
file; however, keeping the two consistent is too much of a hassle, so
I've just expanded the macros into the URDF.

Conflicts:
	urdf/volksbot.urdf.xacro
urdf: changed the radius of the wheels
urdf: changed the way the z-coordinate of base_link is calculated

Note the commit 2d89c0.
Note that the base link is 3 mm heighter than it was before.
urdf: added new link "base_top"

base_top is at the top of the base, above base_link.
Add note about the different wheel radii.