provenance and meshing data from NekMesh
This adds provenance data to the end of an .xml mesh when written by NekMesh. It is the same as fields so gives information on git commit and time etc.
In addition to this when a mesh is made from scratch in NekMesh the meshing instructions are included in this metadata information to give the ability (with git commit id) to reproduce the mesh.
Future updates could add this sort of information to all input classes in NekMesh so that e.g the .xml states that it came from a .msh file and give the name and maybe md5.
Resolves #76 (closed)