Update mesh output for change in Function api

We should use Function.name(), not Function.name.
Use read-only with halo data accessor when outputting data

This removes the need for a separate halo exchange on the Function in
question, and doesn't mark it as dirty.
Pass point data name through PVD call stack

Otherwise, the PVD file doesn't reference the VTUs correctly.
Bump version to 0.5.0
Merge pull request #41 from firedrakeproject/no-halo-in-data-accessor