IMOS check for time_coverage_min/max allows too wide margin of error

[mhidas]

The IMOS checker uses a comparison like np.isclose(time_max, time_coverage_end) to determine whether the global attributes match the range of values in the TIME variable. With the default parameters of np.isclose and typical values of the time variable, this will accept a difference of up to 5.5 hours between the two.

Since the global attributes are specified to the nearest second, the margin of error should be reduced to about 1 second.