Added support for Exodus file writing.

If VTK is present with Exodus support, this commit enables Dolfin to
write out Exodus (.e)-files.
actually add defintion -DHAS_VTK_EXODUS
nest HAS_VTK and HAS_VTK_EXODUS
Unit* -> Unit*Mesh in Exodus tests
merge with HEAD
Merge Exodus file format from Nico Schlömer
  Nico Schlömer 2013-03-07 [merge] merge with HEAD
    Nico Schlömer 2013-02-28 Unit* -> Unit*Mesh in Exodus tests
    Nico Schlömer 2013-02-28 nest HAS_VTK and HAS_VTK_EXODUS
    Nico Schlömer 2013-02-28 actually add defintion -DHAS_VTK_EXODUS
    Nico Schlömer 2013-02-28 Added support for Exodus file writing.
Minor formatting fixes
merge with HEAD
added CMake status message if vtkExodusWriter isn't found
added dolfin::has_exodus() and include Exodus unit tests conditionally on this
Merge:
  Nico Schlömer 2013-03-11 added dolfin::has_exodus() and include Exodus unit tests conditionally on this
    Nico Schlömer 2013-03-11 added CMake status message if vtkExodusWriter isn't found
    Nico Schlömer 2013-03-11 [merge] merge with HEAD
    Garth N. Wells 2013-03-11 [merge] Merge setting matrix block size via TensorLayout.
    Garth N. Wells 2013-03-11 Remove some debug printing.
    Garth N. Wells 2013-03-11 Add block size to TensorLayout and set block size in PETScMatrix.
    Garth N. Wells 2013-03-11 [merge] Merge block matrix fixes and improvements.
    Garth N. Wells 2013-03-11 [merge] Merge with trunk.
    Garth N. Wells 2013-03-10 Fix bug in parallel dof graph builder.
    Garth N. Wells 2013-03-09 Fix some PETSc GAMG option setting.
    Garth N. Wells 2013-03-09 Fix block-size detection function.
    Garth N. Wells 2013-03-09 Add function to figure out dofmap block size.
    Garth N. Wells 2013-03-08 Some fixes for dof blocks in parallel.
    Garth N. Wells 2013-03-08 More work on parallel re-numbering.
    Garth N. Wells 2013-03-08 Fix dof re-ordering in serial and continue work in parallel.
    Garth N. Wells 2013-03-08 [merge] Merge with trunk
Merge:
  Anders Logg 2013-03-11 [merge] Merge:
    Nico Schlömer 2013-03-11 added dolfin::has_exodus() and include Exodus unit tests conditionally on this
    Nico Schlömer 2013-03-11 added CMake status message if vtkExodusWriter isn't found
    Nico Schlömer 2013-03-11 [merge] merge with HEAD
    Anders Logg 2013-03-11 Minor formatting fixes
    Anders Logg 2013-03-11 [merge] Merge Exodus file format from Nico Schlömer
    Nico Schlömer 2013-03-07 [merge] merge with HEAD
    Nico Schlömer 2013-02-28 Unit* -> Unit*Mesh in Exodus tests
    Nico Schlömer 2013-02-28 nest HAS_VTK and HAS_VTK_EXODUS
    Nico Schlömer 2013-02-28 actually add defintion -DHAS_VTK_EXODUS
    Nico Schlömer 2013-02-28 Added support for Exodus file writing.