1extrajoint demo

Demo of the motmod num_extrajoints= parameter

a) XYYZ correspond one-to-one to
   joints 0123
b) Joint4 is an 'extra joint' that is
   not related to any axis letter but
   is managed independently by a limit3
   component.
c) All joints use 'immediate homing'
d) Using extra joints requires
   NON-identity kinematics and all
   joints (including joint4) must be
   homed before switching to world
   mode.
e) A halshow app is opened to display
   key pin values
f) A sim_pin app is opened to set key
   pin values

Usage:
1) F1        (Estop Off)
2) F2        (Machine On)
3) jog joint4 to create a small offset
4) Ctrl-HOME (Home all)
5) demonstrate joint.4 movement using
   the provided M144 mdi command:
   M144 P4 Q1.23 ;move joint.4 to 1.23
   M144 P4 Q3.21 ;move joint.4 to 3.21
6) Alternately, demonstrate joint.[45]
   movement using sim_pin settings:
     j4.limit3.min  min value
     j4.limit3.max  max value
     j4.limit3.maxv max velocity
     j4.limit3.maxa max acceleration
     j4.limit3.in   requested value