Leginon

In cameraclient.py, I had to move the camera parameter retrieval to after the exposure. This is because of the DE12, where we need certain information after exposure (raw frames location, number of frames, etc). As a side affect, the "system time" parameter may be less accurate, even though it was probably not very accurate before anyway. It will now be the time after the exposure and the readout is done.

To solve this problem, I made a new camera parameter "exposure timestamp" which should be the most accurate timestamp that we can get from the camera in pyscope. This is now exposed to leginon, but it needs to use it when doing drift checks and any other time sensitive operations. This change should only be made to leginon when we are sure we can update pyscope on all the scopes, or error checking needs to be put in to fall back to "system time" when "exposure timestamp" is not available.