331 control hand in position

[glannuzel]

Possibility to send goal_position to gripper instead of opening.

Te be used with:

• reachy2-sdk-api : https://github.com/pollen-robotics/reachy2-sdk-api/pull/134
• reachy2_sdk_server : https://github.com/pollen-robotics/reachy2_sdk_server/pull/159

Can be used doing :

reachy.r_arm.gripper.goal_position = 40
reachy.send_goal_positions()