[essimaging] Orca normalization by proton charge per frame

[nvaytet]

We change the way we use the proton charge to normalize the Orca data.

• We normalize each frame in the Dark, OpenBeam, and Sample runs by the corresponding proton charge.
• After that we take the mean of the Dark along the time dimension.
• We then remove the MeanDark from the OpenBeam and Sample data.
• Finally, we divide the (Sample - MeanDark) / (OpenBeam - MeanDark).mean('time')

Fixes #636

[nvaytet]

I had to update the data because the proton charge in the old files started exactly at the start of the time axis for the camera data. However, when doing sc.lookup in 'previous' mode, the first data point was not found (probably because of numerical precision errors) and a NaN was returned.

So the first data slice in the dark lookup was NaN. We then took the mean which gave NaN, and then all data was NaN.
I now updated the proton charge logs to start a bit before the data starts.