Time alignment

[alessandratrapani]

To review after merging #22
Everything should be aligned to the miniscope data recording start time.
Sometimes the first timestamps of the miniscope imaging data are negative.
Thus we need to shift back the recording start time, taking the first negative timestamp as time shift.
When shiting back the recording start time, that is the session start time, all the other traces need to shift forward by the same time shift.

run nwbinspector on nwb file generated with this code version:

[h-mayorquin]

Discussed in person, takes into account the negative shift of the imaging data and shifts everything to take this into account.