Shift timestamps to centre of sampling interval before binning

[mhidas]

For most ADCP instruments the timestamp recorded in our FV01 deployment files refer to the beginning of the sampling interval (which could be 30min or longer). The hourly binning code currently does not account for this. It should shift each timestamp to the middle of the interval before binning.

This may also need to be done for some non-velocity instruments (?)

[mhidas]

This will rely on being able to extract the time-sampling metadata from the file attributes (see #113).