WIP: implement compatibility with yt 3.0

* [ ] get AMR to work (currently stuck with https://bitbucket.org/yt_analysis/yt/issue/890/segmentation-fault-when-using)
* [ ] get octree to work (currently need http://bitbucket.org/yt_analysis/yt/pull-request/1179/fixed-unit-handling-in-load_octree to be merged in and some other changes by Matt Turk)
* [ ] implement converters for polar grids