Remove DagMC class from MOAB namespace

[pshriwise]

The DagMC class is part of the MOAB namespace, but I don't see any reason for it to be. In fact, this is potentially confusing as it seems to act as an extension to the MOAB library (which was the intent some time ago I believe).

I don't think removing the MOAB namespace will have any significant downstream effects outside of DAGMC.

Open to others' thoughts on this!

[gonuke]

Definitely worth looking at. This is most likely a legacy of the origins of DAGMC, but may provide some convenience with not having to reference things explicitly through the MOAB namespace, e.g. error values, etc.