Simplify vertical extrapolation for periodic meshes, ugly hack no longer necessary using picker code.
Simplify vertical extrapolation for periodic meshes, ugly hack no longer necessary using picker code.

rjferrier/fluidity,iakovos-panourgias/fluidity,rjferrier/fluidity,jrper/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,rjferrier/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,rjferrier/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,rjferrier/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity