Project_vtu improvements

Several improvements to project_vtu:

* Add support for P0 (cell-based) fields
* Add support for gmsh. Use .msh filenames directly on the commandline
(identified by .msh extension) instead of triangle file basename.
* Fix (galerkin) projection of tensor fields (also fixed in main Fluidity).
* If the vtu is piecewise linear and continuous, maintain the node ordering of
the donor triangle mesh in the output vtu. This is useful if the output vtu is
used for further analysis with for instance vtudiff.
* Add a test case for project_vtu.
* Update manual and usage message.