Add optical encoders to wheels

[benjie]

To:

• automatically compensate for the Dalek turning down hill.
• give Dalek an active stop state on slopes
• monitor rotation speed and add/remove power as necessary
• automatic power boost to get up hills/over obstacles
• prevent wheel spin
• make acceleration/deceleration smoother

[brackendawson]

We could alternatively use the skirt replacement as a chance to move the wheels forward and one out of two batteries backwards. I'm concerned that retrofitted optical/magnetic encoders are unlikely to be reliable.

[benjie]

We can place the optical encoders where the brakes used to be! That part is very rigid and should be very easy to integrate with.

[benjie]

When building this, make sure we code an odometer! :)