Casmo api

Pull request replacing #73 to account for asymmetric assemblies
I'm not sure why this is tabbed over so far. I think that you may have been using tabs rather than whitespaces. Make sure to make your editor insert whitespaces rather than tabs since this will wreak havoc with Python otherwise: https://mit-crpg.github.io/OpenMOC/devguide/style_guide.html#indentation