Add fixed hierarchy code

[mirenradia]

Chombo's AMR class has this feature which should be implemented here. A suitable tagging criterion should also be written.

[KAClough]

Useful feedback from Dan at Chombo:

I think that right now, if you initialized with a preset hierarchy of grids, it calls the AMR::setupForFixedHierarchyRun, which turns off regridding. However, there’s no really good reason you shouldn’t be able to read in a starting mesh hierarchy and then also allow for regridding later. I think that was simply tunnel-vision on our part when we wrote the original implementations. I will look into removing that at some point, I think.