Ocdsmerge

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Changed
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- The collision behavior is changed to silently ignore the collision, by default.
- Add these flags as possible values of ``collision_behavior``:

- ``ocdsmerge.MERGE_BY_POSITION``: merge objects in arrays based on their array index, instead of their ``id`` value.
- ``ocdsmerge.APPEND``: retain all objects in arrays.

- Remove the ``ocdsmerge.IGNORE`` flag.

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Changed
~~~~~~~

- Warn if multiple objects in an array have the same ``id`` value.
- Add a ``collision_behavior`` argument to ``merge``, ``merge_versioned``, ``add_release_to_compiled_release`` and ``add_release_to_versioned_release``, which can be set to:

- ``ocdsmerge.WARN``: issue a ``DuplicateIdValueWarning`` `warning <https://docs.python.org/3/library/warnings.html>`__ (default)
- ``ocdsmerge.RAISE``: raise a ``DuplicateIdValueError`` exception
- ``ocdsmerge.IGNORE``: silently ignore the collision

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Fixed
~~~~~

- ``get_tags`` no longer returns duplicate tags.

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Changed
~~~~~~~

- Added exception messages to all exceptions.

Fixed
~~~~~

- If there is more than one release, but a ``date`` field is neither a string nor null, the ``NonStringDateValueError`` exception is raised, instead of ``NullDateValueError``.
- If a release is not an object, the ``NonObjectReleaseError`` exception is raised, instead of ``NullDateValueError``.
- If there is a ``TypeError`` for any other reason, it is raised as-is, instead of ``NullDateValueError``.

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- Fix package: Rename VCR cassettes for Windows users.

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- Fix package: Remove ``tests`` from build.