refactor spq_group_by

[maelle]

I am not sure I understand all the steps

[maelle]

I also need more examples. For instance, can one group by a variable created in select.

I know this doesn't work, but maybe for other reasons:

SELECT (COUNT(*) AS ?n) (YEAR(?date) AS ?year)
WHERE{

?film wdt:P31 wd:Q11424.
?film wdt:P577 ?date.

SERVICE wikibase:label { bd:serviceParam wikibase:language "en".}
}
GROUP BY ?year

LIMIT 10