load points from a file.
        returns number of points loaded

def load(self, filename):
        '''load points from a file.
        returns number of points loaded'''
        f = open(filename, mode='r')
        self.clear()
        for line in f:
            if line.startswith('#'):
                continue
            line = line.strip()
            if not line:
                continue
            a = line.split()
            if len(a) != 2:
                raise MAVFenceError("invalid fence point line: %s" % line)
            self.add_latlon(float(a[0]), float(a[1]))
        f.close()
        return len(self.points)