Added materials support by postfixing the material name onto the volume

[jmwright]

No description provided.

[shimwell]

Would it also work if we have a new physical group for each material. Instead of making physical groups a combination of the name and material with the ~ divider. If we have separate physical groups, one for each name and one for each material would that work? I guess it makes it hard to identify which groups are names and which are materials later on so perhaps we can prefix the material physical groups with mat: this is the same prefix we use in DAGMC so perhaps it is a standard hack :-)

[jmwright]

@shimwell I've made that change to attach the material name via a physical group.