Add TIME_bounds variable to describe binning intervals #113

There are CF standard ways to describe values that are representative of an interval, and we should be using these to document our products. In particular, binning time intervals can be described using a TIME_bounds variable. We should add such variables in

• The aggregated products, to describe any binning that has already been applied to values in the input files
• The hourly products to describe the hourly binning intervals.

Ideally, the bounds relevant to the input files would just be copied from those files, but they are not currently in there. However, they can be computed from existing metadata in the files (such as the TIME:seconds_to_middle_of_measurement attribute in ADCP files, or the instrument_burst_duration global attribute).