Add optional tip_position annotation

[alejoe91]

While the tip position is implicitely contained in the probe contour, it could be helpful, especially for some projects like brainreg, to have an explicit annotation/property with that info

[samuelgarcia]

bonne idée.
What to do when multi shank ? Do we have a list always ?

We could regenerate the actual library by taking the min(contact_y) ?