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 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). 
This patch will fix this problem.
can you please add a newline before ```spherical_position[1]=0.;``` and a comment that explains what you are doing?