brittonsmith/ytree: ytree 2.2.1 Release

ytree version 2.2.1 is released! ytree is an extension of yt for working with merger-tree data from multiple sources. This is a bugfix release with a couple of enhancements that made some of the bugs easier to fix. Enhancements Refactor of CatalogDataFile class. Simplify requirements file for docs build on readthedocs.io. Bugfixes Restore access to analysis fields for tree roots. Fix field access on non-root nodes when tree is not setup. Fix issue of uid and desc_uid fields being clobbered during initial field access. ytree can be installed with pip: pip install ytree or from source, by going to https://github.com/brittonsmith/ytree git clone https://github.com/brittonsmith/ytree cd ytree pip install -e . Full documentation can be found at http://ytree.readthedocs.io/