Climate-categories

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* Change conversion metadata format to use comment chars and a YAML header.

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* Use Python files instead of pickle objects for caching

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* Export Category and HierarchicalCategory types.
* Add ConversionRule.is_restricted attribute to easily check if a rule is restricted to
specific auxiliary categories.

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* Export Conversion and ConversionRule types.

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* Add emissions categorization from the `Reduced Complexity Model Intercomparison Project (RCMIP) <https://www.rcmip.org/>`_. Thanks to Robert Gieseke for the contribution and Zeb Nicholls for input.

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* Automate changelog generation from snippets - avoids resolving merge conflicts
manually
* Automate github releases.
* Add category "0" (National total) to IPCC1996 and IPCC2006 categorizations. While it
is not in the official specification, it is widely used and adding it also enables
automatically assigning a level to all other categories.
* Add categorization CRF1999 used within in the common reporting framework data.
* Refactor rendering of large categorizations using ``show_as_tree()``, adding more
clarity to alternative child sets. Add usage documentation for ``show_as_tree()``.
Thanks to Robert Gieseke for feedback.
* Fixes for IPCC2006 categorization (and IPCC2006_PRIMAP):

- proper title for category 3.B.3.a "Grassland Remaining Grassland"
- correct corresponding 1996 category for category 1.A.4.c.ii

* Fixes for IPCC1996 categorization:

- category 4.B.10 has the correct title "Anaerobic Lagoons"
- correct usage of units in the titles of categories 4.C.3.a and 4.C.3.b

* Add mechanism to describe conversions between categorizations.
* Add conversion between IPCC2006 and IPCC1996.
* Add algorithm to detect over counting in conversions between categorizations.
* Refactor generation of IPCC2006 and IPCC1996 categorizations.
* Add function to find unmapped categories in a conversion.