Experimentator

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- Give experimentator a `DOI <https://zenodo.org/badge/latestdoi/22554/hsharrison/experimentator>`_.

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- Change the save format from pickle to YAML. This allows experiment changes to be diffed.
- Unify run callbacks and context managers. Now |Experiment.add_callback| takes an optional argument ``is_context`` to differentiate the two.

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- Fix bug that occurred when using |Sorted| caused by conditions being a generator object.

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- Add |ExperimentSection.description| property.
- Reduce number of debug messages when running sections.
- Fix bug where parent sections would be marked as having started after running demo trials.
- Workaround for pandas bug (issue 7380) where comparing two different DataFrames would raise an exception instead of returning ``False``.
- Comparing any of experimentator's objects to a different type now returns ``False``.

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- Allow tuple indexing.

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- Report coverage to coveralls.
- Add contact request to README.
- Make badges prettier.