Handling for lbtim.ib=3 intervals

Currently the IA (interval in hours) value for time aggregated data in PP files is ignored when IB is 3 (aggregation over a period within a year over a series of years). This PR modifies the behaviour to add the appropriate cell methods. One issue I am unclear on (even having spoken to a number of people) is how far the LBPROC value propagates. For example, if LBPROC = 8192 (maximum over period, rather than minimum or mean), does that imply the result is the maximum over the period across a series of years (max of max if you like), or is it the average (mean) over a number of years of the maximum within each period in each year. IN other words, say the period is January and the years are 2000 to 2009 (inclusive) - is it the maximum temperature across all ten Januaries (Januarys?) or is it the average (mean) of the maximum temperature in each January? Any thoughts welcome.
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