Implement a way of setting up the depth live

[theelims]

Right now the lib has no feature to set up the depth and get a live feedback. It can only go full in or full out. However, it would be desirable to have it follow the depth setting to find an optimal position before starting the stroking.

[theelims]

In a special setup mode it will always follow the Depth position. By envoking Stroker.setupDepth(); it will start to follow the depth position whenever Stroker.setDepth(float); is updated. Takes the speed in mm/s as an argument: e.g. Stroker.setupDepth(10.0); Speed defaults to 10 mm/s. With float Stroker.getDepth() one may obtain the current set depth to calculate incremental updates for Stroker.setDepth(float). Can be called from states SERVO_RUNNING and SERVO_READY and stops any current motion. Returns false if called in a wrong state.