tests: Add test_mpi_advection_diffusion.cpp.

This implements a basic test for the MPI-PFASST version of the
advection/diffusion example.
Merge branch 'development' into feature/test-mpi
tests: Update expected solutions and tighten tolerances.
test: Loosen tolerances.
Merge pull request #57 from memmett/feature/test-mpi

Add test for MPI version of advection/diffusion example.  This closes #41 as well.