Prepare for processing two-dim. grid

Allow a two-dimensional grid to be sent to compute_geometry() without
the assertion triggering. Route three-dimensional grids to the existing
implementation.
Compute properties for edges in 2D
Compute properties for cells in 2D
Set size properties in grid when allocating structure

There is little chance that the client want to set them otherwise since
that would make the structure inconsistent, and having all the clients
do the work is just needless duplicate labour.
Unit test the geometry computations for 2D grids
Merge pull request #121 from rolk/compgeo2d

Compute (extra) geometry properties for 2D grids