add pre eval callback and stringify user info for logging

[LGro]

• Close JSON dump in optimization loop logging fails for some user_info values #97 by casting the user_info to string before dumping it to JSON for logging
• Add pre-evaluation callback
• Add reference to evaluation function wrapper in docstring for more transparency about catch_exceptions_from_evaluation_function argument

[github-actions]

Coverage report

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The coverage rate is 89.54%.
The branch rate is 87%.

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blackboxopt/optimization_loops/sequential.py

100% of new lines are covered (89.58% of the complete file).

[dynobo]

Looks fine, I'm only curious, for which use-cases this feature is used. :-)

[sfalkner]

Thanks @LGro. These changes make the optimization loop more flexible 👍