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)'], ...)