Codegen operation fix args validation, add docstrings

[barbieri]

This PR improves the operations and schema code generators:

• schema may contain docstrings, closes #122
• operations: revert #123, seems to work properly, otherwise report again, closes #133
• result interpretation throws errors or make them easily available, closes #141

I'd like some testing so I can release v13 soon.

[coveralls]

Pull Request Test Coverage Report for Build 207

• 16 of 16 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 199:	0.0%
Covered Lines:	1483
Relevant Lines:	1483

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💛 - Coveralls

[barbieri]

released as https://pypi.org/project/sgqlc/13.0/