This simple test program demonstrates how to implement a PDE solver
with DOLFIN.

The problem first needs to be stated in variational form: Find u in V
such that

    a(v, u) = L(v)    for all v in V.

DOLFIN uses the FEniCS Form Compiler FFC to process variational forms.
The bilinear form a and the linear for L are here specified in the file
Poisson.form which can be compiled using the command

    ffc Poisson.form

to create the file Poisson.h included in the main program.

The simple test program saves the solution in VTK format to the file
poisson.pvd, which can be visualized in ParaView or MayaVi. A simple
way to visualize the solution is provided by the plot.py script:

    python plot.py

Note that there is also a ready-made Poisson solver module in DOLFIN.
A simple demo program for this solver is available in the subdirectory
src/demo/solvers/poisson/ of DOLFIN.
