BUG/DOC: Categorical fixes (GH7918)
Categoricals now raise NotImplementedError when writing to HDFStore with a Fixed type store
Slicing bug with a single-dtyped category and a possible view

closes #7918

https://github.com/pydata/pandas/issues/7621 will fix this in a future version (or if someone gets to it sooner). Its not very hard to actually write the categorical at all, the issue is retaining the meta-data (e.g. levels) needs to be done for recreation (and query).
 cc @JanSchultz
 seems good!
