Make Dat._data temporarily writable for halo exchange
Add MPI advection-diffusion demo
Add output testing for MPI advection-diffusion demo
Add profiling option for MPI advection-diffusion demo
Add MPI advection-diffusion regression test

Pickled mesh data is automatically downloaded by the test script.
Merge pull request #156 from OP2/mpi-advection-diffusion-demo

MPI advection diffusion demo and regression test

Adds an MPI advection-diffusion demo and regression test. Since we do
not have an easy way of generating a decomposed mesh and halos for a
complex MPI test case, the input data is provided in form of a gzipped
pickle dump which is automatically downloaded by the regression test and
read in the demo.