Draft: Multiple timing constraints in the add method of schedule

Explanation of changes

• Handling several timing constraints in the add method of Schedule.
• Deleting changes from !733 (merged)
• Set default value of ref_schedulable in Schedule.add instead of doing it in determine_absolute_timing.

Motivation of changes

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It is already possible to add several timing_constraints to a Schedulable of a Schedule. However it is currently unintuitive, timing constraints are added with 2 different methods (Schedule.add and Schedulable.add_timing_constraint) which do not have the same argument names (ref_op in the former is ref_schedulable in the latter). Handling several timing constraints directly in Schedule.add would make it easier to build schedules.

Closes #422

Merge checklist

See also merge request guidelines

• Merge request has been reviewed (in-depth by a knowledgeable contributor), and is approved by a project maintainer.
• New code is covered by unit tests (or N/A).
• New code is documented and docstrings use numpydoc format (or N/A).
• New functionality: considered making private instead of extending public API (or N/A).
• Public API changed: added @deprecated (or N/A).
• Tested on hardware (or N/A).
• CHANGELOG.md and AUTHORS.md have been updated (or N/A).
• Windows tests in CI pipeline pass (manually triggered by maintainers before merging).
  • Maintainers do not hit Auto-merge, we need to actively check as manual tests do not block pipeline

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