AssemblyContext

This PR adds AssemblyContext, which subclasses FEMContext. Right now, it doesn't do anything, but it gives a place to do work in moving CachedValues, etc. as described #27. I redid the internal GRINS API's to take an AssemblyContext. Right thing to do later will be to remove CachedValues from the API and instead grab it from the AssemblyContext. One thing at a time though. Will wait a day or two to merge this in case this borks someone.