'get the repatition of rating grade for the given res_ids.
:param domain : optional domain of the rating to include/exclude in grades computation
:return dictionnary where the key is the grade (great, okay, bad), and the value, the number of object (res_model, res_id) having the grade
the grade are compute as    0-30% : Bad
31-69%: Okay
70-100%: Great'

@api.multi
def rating_get_grades(self, domain=None):

    data = self.rating_get_repartition(domain=domain)
    res = dict.fromkeys(['great', 'okay', 'bad'], 0)
    for key in data:
        if (key >= 7):
            res['great'] += data[key]
        elif (key > 3):
            res['okay'] += data[key]
        else:
            res['bad'] += data[key]
    return res
