Returns minimal positive value or None.

def min_pos(self):
        '''Returns minimal positive value or None.'''
        if self.__len__() == 0:
            return ArgumentError('empty set has no minimum positive value.')
        if self.contains(0):
            return None
        positive = [interval for interval in self.intervals
                    if interval.left > 0]
        if len(positive) == 0:
            return None
        return numpy.min(list(map(lambda i: i.left, positive)))