Modify the the lagrange interpolation function described in class
in order to produce the bivariate version.
that is given a matrix 
A = Matrix(ZZ,[[1,2],[3,4],[5,6]])
complete the sage code for the 
following lagrange interpolation 
function such that we have
 
g = bivarite_lagrange_interpolation(A,3,2)
g.subs(x=0,y=0) = 1
g.subs(x=0,y=1) = 2
g.subs(x=1,y=0) = 3
g.subs(x=1,y=1) = 4
g.subs(x=2,y=0) = 5
g.subs(x=2,y=1) = 6

def bivarite_lagrange_interpolation(A, m, n):
    var('x,y')
    f = 0
    # Insert your sage code here
    return f