Purpose of this commit is to give description more "business oriented"
because those descriptions appears in Odoo Studio which is supposed to be used by end users, not only by developers.

Related Task ID : 37311

This commit add an extra parameter 'fill_temporal' to read_group which allows
the orm to add missing groups for date intervals. This is useful for charts.

Suppose that we are in a use case where data are grouped by a date fields
(typically months but it could be another interval) and displayed in a Bar Chart
or a Line Char.
Let's says a request has to group records by month for August, September
and ...December.  If we don't changed anything, we would get a Bar Chart
looking like this :
                                        ___
                              ___      |   |
                             |   |     |   |
                             |   | ___ |   |
                             |   ||   ||   |
                             |___||___||___|
                              Aug  Sep  D
December follows directly after September, it can be unintuitive for the
user, so we change that. We add some fake records for each missing months
between the earliest and the lastest date of the result
                                             ___
                         ___                |   |
                        |   |               |   |
                        |   | ___           |   |
                        |   ||   |          |   |
                        |___||___| ___  ___ |___|
                         Aug  Sep  Oct  Nov  Dec

This commit is part of task #1835644

The caller of `read_group` can now provide the aggregating function to use for
a given field:

    # set aggregating operator for fields 'foo' and 'bar'
    model.read_group(domain, ['foo:sum', 'bar:avg'], ...)

One can also aggregate the same field several times, by giving a specific
output name for each:

    # aggregate 'foo' with both 'min' and 'max'
    model.read_group(domain, ['foomin:min(foo)', 'foomax:max(foo)'], ...)

The attribute `group_expand` allows to reorder and add empty groups to the
result of `read_group`.  Enable this feature for other fields than many2one
fields.