feat: add validate_annotations_consistency function

[PeterStaar-IBM]

Validating self-consistency of annotations

There are a set of simple rules that can be checked and ensure consistent annotations

        def _validate_to_captions():
            """validate to captions.

            - Make sure the first bbox is a FloatingItem and the subsequent
            are of type caption.
            - Make sure that the to_caption only starts at the beginning of
            a group, if a group is present.
            """
            return True

        def _validate_to_footnotes():
            """validate to footnotes.

            - Make sure the first bbox is a FloatingItem and the subsequent
            are of type footnote.
            - Make sure that the to_caption only starts at the beginning of
            a group, if a group is present.
            """
            return True

        def _validate_captions():
            """validate captions.

            Make sure each caption either is the start of a to_caption or
            has a merge node.
            """
            return True

        def _validate_list_items():
            """validate list items.

            Make sure every list-item has a group or merge point.
            """
            return True

        def _validate_reading_order():
            """validate reading-order.

            Make sure that every bbox has a reading-order point or is
            entirely overlapped with a bbox that has a reading-order.
            """
            return True

        def _validate_merges():
            """validate merges.

            Make sure that all bbox in a merge are of same type
            """
            return True

        def _validate_group():
            """validate groups.

            Make sure that all bbox in a group are of same type
            """
            return True

[mergify]

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