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
Merge pull request #136 from bska/release-cleanup

Avoid clobbering existing tests for SuperLU
Use portable method of zeroing vector of ints.

The memset() technique is only applicable to platforms for which
numerical zero is represented by all bits zero.
Eliminate release-mode build warning.

The 'cbottom' variable is only used within an assert().  Don't define
the variable in release (i.e., "NDEBUG") mode.
Eliminate redundant explicit type conversion.

The pointers in question are already type 'int *'.  There is no need to
explicitly convert them to that type too.
Replace an assignment with intended equality test

This corrects a latent error that has been present since the inception
of this module.