Write categories.xml

We add the ability to write categories.xml files through Writer.addxml('categories', **kwargs). We also expand parsing of categories.xml to parse key elements (these can also be written).

We incorporate the cached_property dependency into the mffpy library. The rationale for this was using cached_property_bel as a dependency caused mffpy to clash with other packages requiring cached_property as a dependency.

Finally, we add a CHANGELOG.md to track changes made in each release.