Invalidate the most recent Buffer State Update when a new Buffer System Description comes in.

[jpwijbenga]

When a new system description (with different fill level functions and ranges belonging to those fill level functions) comes in, the agent takes the old fill level (which is not appropriate anymore, because the system has changed).
So the following situation may occur: A bid is made when the new sys description comes in, but the agent asks for the current fill level. The fill level for the old sys description is returned and this may not match the range of the new fill levels.

Proposed fix: clear and undo the latest state update when a new sys description arrives, because it is not valid anymore. Check all helper methods that they will throw an exception instead of return a false value.