multi_step_cov_estimator: added 'default_initial_pose' parameter

This will replace the previously used value [0 0 0 0 0 0] as initial pose guess by an (optional) parameter placed in the system.yaml. On dewey, the calibration woudn't converge otherwise.

Not sure about this. Maybe this should go into the 'checkerboards' section.

Example system.yaml:

	base_link: base_link
	
	sensors:
	
	  chains:
	    torso_chain:
	      root: base_link
	      tip: head_tilt_link
	      cov:
	       joint_angles: [0.01, 0.01, 0.01, 0.01, 0.01]
	      gearing: [1.0, 1.0, 1.0, 1.0, 1.0]
	
	  rectified_cams:
	    head_camera:
	      chain_id: torso_chain
	      frame_id: head_camera_rgb_optical_frame
	      baseline_shift: 0.0
	      baseline_rgbd: 0.075
	      f_shift: 0.0
	      cx_shift: 0.0
	      cy_shift: 0.0
	      cov: {u: 0.25, v: 0.25, x: 0.25}
	    base_camera:
	      chain_id: NONE
	      frame_id: base_camera_rgb_optical_frame
	      baseline_shift: 0.0
	      baseline_rgbd: 0.075
	      f_shift: 0.0
	      cx_shift: 0.0
	      cy_shift: 0.0
	      cov: {u: 0.25, v: 0.25, x: 0.25}
	
	  tilting_lasers: {}
	
	checkerboards:
	  cb_7x6:
	    corners_x: 7
	    corners_y: 6
	    spacing_x: 0.108
	    spacing_y: 0.108
	
	transforms:
	  dummy_transform:   [ 0, 0, 0, 0, 0, 0 ]
	
	default_initial_pose: [ 3, 0, 0, 0, 0, 0 ]