only fully empty and refill syringe when needed

[kaeverens]

at the moment, we empty and refill the syringe every time the robot returns to base.

this is a waste of time if we only used a little water and there is still enough left in the syringe to do the next mission.

the robot should be changed so that

1. it empties/refills when the robot is booted.
2. it records how much water is in the syringe after each action
3. the next empty/refill is done only if a mission would reduce the syringe to below 0ml

[kaeverens]

done!