Fix a bug that gives wrong velocity near the poles.
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Merge pull request #227 from SiqiZhang/gplate_fix

Fix a bug in gplate plugin: While gplate uses zero longitude at all grid points at poles, we assume the longitude at those points vary from 0 to 360. This results in a wrong velocity to be interpolated near the poles (the velocity between poles to the first latitude grid points would be wrong, however the velocity right at the pole will be OK).