Add support to group by year of date properties

For some content, the most pertinent grouping would be by a date property.

Here we implement a bare-bones version of it, with a new template parameter
triggering a grouping by year: the date is then converted using YEAR.

Ideally, we would be able to specify another granularity than year (eg decade/century).
This is not as straightforward (as far as I know, no handy SPARQL primitive like YEAR)
so let's kick that use case down the road.

• Add GroupingType Enum
• Add grouping_type class attribute to PropertyStatistics
• Add BIND dance to:
  • get_grouping_information
  • get_query_for_items_for_property_negative
  • get_query_for_items_for_property_positive
• Change accordingly the formatting of groupings in make_stats_for_one_grouping
• Add unit tests

Bug: T236590