Graphtransliterator

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* Added test to check compressed dump is uniform
* Fixed sorting of class id in compressed dump to make JSON output uniform
* Added Python 3.8 support

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* Added pre-commit hook to rebuild bundled transliterators with bump2version
* remove to_dict from DirectedGraph, since it is handled through Marshmallow schemas.
* Adjust documentation to mention compression.
* added list-bundled CLI command
* added --regex/-re flag to graphtransliterator make-json CLI command to allow regular
expressions
* removed coverage keyword from GraphTransliterator
* reorganized core.py
* converted from_dict, from_easyreading_dict, from_yaml, and from_yaml_file to static
methods from class methods
* moved ambiguity-checking functions to ambiguity.py and tests to test_ambiguity.py
* set three levels of compression: 0 (Human-readable), 1 (no data loss, includes graph),
2 (no data loss, and no graph); 2 is fastest and set to default.
* set check_ambiguity to read keyword during JSON load
* allowed empty string productions during JSON compression
* added compression.py with decompress_config() and compress_config() to compress JSON
* added tests/test_compression.py to test compression.py
* added sorting of edge_list to DirectedGraph to allow dumped JSON comparison in tests
* adjusted _tokenizer_string_from() to sort by length then string for JSON comparison

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* added IncorrectVersionException, if serialized version being
loaded is from a later version than the current graphtransliterator
version
* added automatic edge_list creation if edge parameter in DirectedGraph
* added fields to and started using NodeDataSchema
* added pre_dump to GraphTransliteratorSchema, NodeDataSchema to remove empty values
to compress Serialization
* removed rule from graph leaves and updated docs accordingly

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* fixed serialization of graph node indexes as integer rather than strings

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* added JOSS citation to README
* added --version to cli
* removed some asserts
* removed rule dictionaries from graph leaves to compress and simplify serialization

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* updates to docs