Grokcore.catalog

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- Introduce Datetime index that's more optimized for index datetime objects.
Please note the index uses seconds-resolution (the integer timestamp
representing the datetime's value).

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- Make possible to install a catalog in a site on a different event
than ``IObjectAddedEvent`` using the ``grokcore.site.install_on``
directive.

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- Rename IIndexDefinition and IndexDefinition into IAttributeIndexDefinition
and AtributeIndexDefinition respectively. This is used for the current
index "classes" and allow for setup() magic for attribute indexes when
creating new catalogs.

This allows for IIndexDefinition and IndexDefinition to be used for
simpler catalog index definitions where no Grok magic is applied when
creating catalogs.

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* Initial fork from Grok.