Construct a "proper" SGRID mesh

[ocefpaf]

The current implementation of gridgeo does not construct a proper sgrid object, only a simple quad grid using the variable coordinates data. (One might argue that is the "correct" grid when dealing with a particular varibable.)

It would be interesting to implement a light-weight parser for sgrid, like the one for ugrid that preserves the knowledge of grid centers and cornersand create a proper sgrid mesh from them.