3D On sphere

The 2D mesh can be extruded radially to form a 3D spherical mesh. The 3D equations can be solved on a sphere. A new benchmark is added (test the geostrophic balance on the sphere). The tracer equations can be solved independently in 3D. A heat flux can be used as bottom/boundary condition for the temperature equation (in 3D). The user can add a surface pressure term which is a function of eta and deta_dt. The user can add a surface friction term in SLIM3D. The gravity field can spatially vary. The restart function works for spherical meshes.