... and xacro:panda_arm -> xacro:arm
to be consistent with hand.xacro

IMO, prefix much better describes the purpose of these two variables
in the present context, namely providing a name prefix to joints and links.

They serve different purposes:
- coarse collision geometries mimic the internal self-collision checking
  of the real-robot controller
- mesh models should be use for collision checking with environmental objects

- Declare inertial properties in a yaml file
- Use xacro:inertial macro

... in the center between and aligned with the fingers

…t-states-desired) and frankaemika#194 (tool-frame) into develop