Authorityspoke

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* Create `ComparableGroup` class for unordered `Factor` collections
* Create `FactorSequence` class for ordered `Factor` collections
* Eliminate Analogy class, moving its methods to `FactorGroup` and `FactorSequence`
* Add `Factor.consistent_with` method to search for available context avoiding contradiction
* Add "or" operator for `FactorGroup`
* Integrate Legislice API client for retrieving text from US Constitution and US Code
* Remove functions for loading legislation text from XML files. Use API client instead.
* Delete classes for accessing XML legislation files: `Code`, `Regime`, and `Jurisdiction`

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* Create broader conditions for Procedure.contradicts()

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* Add `__init__.py` to utils folder

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* Publish repo's utils folder as part of AuthoritySpoke package

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* Fix bug where some types of cross-references caused loading of Holdings from JSON to fail
* Update case download function because Case Access Project API no longer includes "casebody" field in all responses from cases endpoint
* `new_context` function can use string to find Factor to be replaced
* Enactment URIs can target a chapeau or continuation
* Fix bug that created [multiple pint Unit Registries](https://github.com/hgrecco/pint/issues/581)

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* Enactments may choose text by section without a TextQuoteSelector
* Remove "regime" parameter from Enactment
* Add data serialization using [Marshmallow](https://marshmallow.readthedocs.io/)
* Migrate JSON data loading functions to Marshmallow
* Add Decision class containing Opinions
* Add Explanation class to clarify relationships between Holdings
* Improve readability of string representations of objects
* Move text selectors to separate [anchorpoint](https://anchorpoint.readthedocs.io/) library
* Add [apispec](https://github.com/marshmallow-code/apispec) schema specification for Holding input JSON files