Leginon

r17334 added an image acquisition without using it for correlation so that the shutter is activated once. It seems to remove enough hysteresis to eliminate the problem.

A cause is also added to prevent singular low drift to exit the monitor loop when the drift rate jumps around. Example of the drift rate sequence that could exit in the old method is (...,3.0,0.8,0.2,1.4) If the threshold is 1.0, then the last two average will be 1.3 and 0.8.