This commit provides 1 more field on the rating mixin: the average of
the ratings of the document. Before that, the only way to get the
average was to call `rating_get_stats` record per record.
As we are on a mixin, this new field is not stored to avoid recomputation
each time a rating is added. To improve performance, we decided to
compute `rating_count` and `rating_avg` in one `read_group`.
This commit also set an extendable method to get the domain of the
rating to include in those statistics computation, called `_rating_domain`.
This way, each model inheriting the rating.mixin can define the subset of
pertinent ratings.
As consequence, this commit uniformized the way average is displayed:
instead of taking the closer value to the 0.5 value, it only round it
at 2 digits.
Task-1902304
jem-odoo
8c840f7539