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 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) (done 09/2003)
 Generalize relaxation: (done 11/2002)
* necessary for CrankNicholson
* " for variable density (done 11/2002)
 GFS > OpenDX file conversion (done 11/2002, DX module).
 Optimize parallel code by overlapping communications and
computations: (done 2009)
This could be done relatively easily (in Poisson relaxation for
example) by first relaxing the cells close to the boundaries, start
the communications, relax the remaining bulk cells then synchronize
the communications.
