Fix calc of centre of "local" station/event plots.

When the centre of the station/event plot is outside ±90, the sum of the
min&max are greater than ±180. The Longitude class does not like this
value, but this can be fixed by halving both before the addition.
Merge pull request #814 from QuLogic/fix-local-plot-range

Fix calc of centre of "local" station/event plots.