WIP: feature/meshgraph-factory
Sorry Dave this is a big MR.
This does a bit of a cleanup of spatial domains, specifically the way meshgraph deals with composites which was confusing and a bit convoluted.
There is a change in the meshgraph in that it is now a proper factory, at this time it is one ended, i.e only XML. But this does remove the 1D 2D 3D separation which isnt needed and should make it easier to put HDF5 geometries in soon.
This MR is dependent on all the C++11 + cmake related MRs.