Calculate the nth order discrete difference along given axis.
The first order difference is given by out[n] = a[n+1]  a[n] along the given axis, higher order differences are calculated by using diff recursively.
Parameters :  a : array_like
n : int, optional
axis : int, optional


Returns :  diff : ndarray

Examples
>>> x = np.array([1, 2, 4, 7, 0])
>>> np.diff(x)
array([ 1, 2, 3, 7])
>>> np.diff(x, n=2)
array([ 1, 1, 10])
>>> x = np.array([[1, 3, 6, 10], [0, 5, 6, 8]])
>>> np.diff(x)
array([[2, 3, 4],
[5, 1, 2]])
>>> np.diff(x, axis=0)
array([[1, 2, 0, 2]])