Fix time_coverage_resolution in add_range_attrs (set based on data, not 'variable')

[oyvfoss]

In nc_format.conventionalize.add_range_attrs, we are currently setting time_coverage_resolution to variable.

This is desirable for something like a CTD dataset, but for something with fixed (or nearly fixed) sample interval, like a mooring sensor, we should set this based on the data.

The line in question (L74 in nc_format.conventionalize in current commit)

        D.attrs['time_coverage_resolution'] = 'variable' 
        ## Note: Should not be variable for fixed sampling rate instruments!!! -> Fix this.

Notes:

• May have to think about a criterion for "fixed interval"..
• May have to think about data gaps; seems we could allow for gaps but still say that here is a "time_coverage_resolution"..

[oyvfoss]

Fixed in 6f7a984687c0fb25fba1233f10910ab4731857ce.