Conference-scheduler-cli

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* Bug Fix: Pre-allocated events were being scheduled in addition to their
pre-allocated slot when optimising for consistency

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* Minor: Order of fields altered in schedule.csv

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* New Feature: --diff option to control logging of schedule diff

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* Bug Fix: Consistency optimisation fixed

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* Minor: Updated log message for Unavailability

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* Bug Fix: Remove use of strings for Slot.starts_at

* New Feature: Unavailability and clashes can now be defined for events as well
as people

* New feature: Capacity objective now split into efficiency or equity