Make retractable more parametric

I needed a longer probe to work with my Mk IV J-Head. So I'm using a
2.5mm hex now instead of the 1.5mm that Johann uses. To support that, I
modified the design of the retractable and made it more parametric in
the process.
M3-based probe clamp

This clamps the probe at the end of the spring, because I didn't have
anything else on hand to use for the purpose.