Cffconvert

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- added schema.org converter method

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- added documentation for the Google Cloud Function interface

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- setup.py no longer includes test dependencies as install dependencies

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- corrected an error where cffconvert could not raise an error during validation (https://github.com/citation-file-format/cff-converter-python/issues/94).

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- replaced pykwalifire with its parent pykwalify
- now works for python 3.7 (refs 80)
- not using PyYAML anymore (but it still comes along with pykwalify for some reason)
- added a function that can be used as Google Cloud function
- hopefully fixed parsing of strings that should have been entered as dates (the new validator does
not find that offensive, hence I had to fix it myself)

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- replaced PyYAML dependency with ruamel.yaml