Lbtim3 time point

Changes the time point for the climatology bounds of a PP field to be the mid-point between the first two bounds.

For instance, supposing there were a climatology bound for ```"1960-3-1",  "1990-6-1"``` then the sample point would be ```"1960-4-16"```.

This is not the final state of play for the ```climatology_bounds``` concept, but it fixes the erroneous time point which is currently being used. For discussion see https://groups.google.com/forum/?fromgroups=#!topic/scitools-iris-dev/8rjP3Mf3iMM