delta: Support limiting the maximum velocity of z moves

On a delta printer, z moves require the mcu to support the greatest
number of steps per second.  However, z moves are rare, so it makes
sense to limit the velocity of z moves separately from the velocity of
normal xy moves.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-delta.cfg

@@ -79,8 +79,13 @@ pin_map: arduino
 [printer]
 kinematics: delta
 #   This option must be "delta" for linear delta printers
-max_velocity: 200
+max_velocity: 300
 max_accel: 3000
+max_z_velocity: 200
+#   For delta printers this limits the maximum velocity (in mm/s) of
+#   moves with z axis movement. This setting can be used to reduce the
+#   maximum speed of up/down moves (which require a higher step rate
+#   than other moves on a delta printer).
 delta_arm_length: 333.0
 #   Length (in mm) of the diagonal rods that connect the linear axes
 #   to the print head