'Add sort condition for cpecified range of cells.
:param ref: range of the cells (e.g. \'A2:A150\')
:type  ref: string
:param descending: Descending sort order (default=``False``)
:type  descending: bool'

def add_sort_condition(self, ref, descending=False):

    sort_condition = SortCondition(ref, descending)
    self._sort_conditions.append(sort_condition)
    return sort_condition
