Feature design: safety features

[JannesHN]

The current version has an IR proximity sensor to check whether the lid is closed. ..and we took that off because it was too flimsy, so we have not safety feaure at all right now.

Things that we should be adding:

• a simpler sensor for the closing of the lid.
• a more robust lid. Not PMMA, but metal one would be nice.
• a system that checks wether the rotor is well balanced and can stop the spinning when it's not.