'**SUMMARY**
Returns the y intercept based on the lines equation.  Note that this point is potentially not contained in the image itself
**RETURNS**
Returns a floating point intersection value
**EXAMPLE**
>>> img = Image("lenna")
>>> l = Line(img, ((50, 150), (2, 225))
>>> b = l.getYIntercept()'

def getYIntercept(self):

    if (self.yIntercept is None):
        (pt1, pt2) = self.end_points
        m = self.slope
        self.yIntercept = (pt1[1] - (m * pt1[0]))
    return self.yIntercept
