Safe movements

[romainbrette]

Currently we only specify the target of movements, not the path. This could lead to collisions, or to inappropriate movements in tissue.
The basic feature should be: any manipulator movement should be done first on the focal plane, then along the X-axis. Later we might want to implement collision detection, in particular between pipettes. Finally speed should be controlled: currently it is set at the fast value. Instead we might want to have a decreasing speed, or use a speed proportional to the magnitude of the movement to be performed.

[romainbrette]

This is partially implemented, except for speed control.
The MP moves first on a horizontal plane, then along its X axis. One check remains to be implemented: that the intersection of X axis and horizontal plane is within reach.