- Add corner flow test case.

- The boundary conditions for the gradient of the pressure could be
improved.

Possible speed optimizations: 

- packing of parallel communications (in the case of domain composed
of several boxes).  

- parallel match() uses doubles for cell layout description, guint
would be more space efficient.

- multiple calls to fl_domain_depth() are not needed => store maxlevel
in FlDomain struct and update it only when necessary (i.e. when
refining/coarsening etc...).

Dodgy stuff:

- Check that gfs_face_gradient() is used properly when not used for
  poisson problem (i.e. advection) (particularly in 3D with the factor of 2).

Future stuff:

- Richardson extrapolation => being able to compute advection terms
  one level above the leaf cells.

- Adaptation given a constraint on maximum domain size (i.e. optimal
  distribution of a finite number of cells).

- Generalize relaxation: (done 11/2002)
  * necessary for Crank-Nicholson
  *    "      for variable density (done 11/2002)

- GFS -> OpenDX file conversion (done 11/2002, DX module).

