Manual calibration improved : now as precise as last auto calibration

[renarded]

• User needs first to do an auto calibration.
• The absolute (sensor) steps are saved on the RPi3
• User can know use manual calibration, by placing the robot at "almost" home position
• Absolute (sensor) steps will be read and will enable to calculate the offset so the manual calibration has the same precision as the last auto calibration

--> Saves time (~30 s) and reduces the wear of mechanical parts