First work on Mesh class

This PR establishes a Mesh class where unstructured and structured iMesh instances can live side-by-side. The main reason for doing this is that many other classes that will inherit from ```Mesh``` (e.g. ```Meshtal```) need both structured and unstructured capabilities. From a glance around the internet I do not think dynamic inheritance is particularly Pythonic.

I refactored the ScdMesh class created by Steve Jackson, retaining all functionality, and put it within the ```Mesh``` class. I created an ```__init__``` to accommodate 6 different combinations of arguments for instantiating unstructured/structured mesh, detailed in the class doc strings.

Future work will include mesh arithmetic methods (with operator overloading) and corresponding duck-typed methods within a ```StatMesh``` class.

With this PR, ```mesh``` merging with ```staging``` is nigh.