replace ''\N" with np.nan throughout dataframe code example

Example 1: replace nan in pandas

df['DataFrame Column'] = df['DataFrame Column'].fillna(0)

Example 2: pandas replace nan with value above

>>> df = pd.DataFrame([[1, 2, 3], [4, None, None], [None, None, 9]])
>>> df.fillna(method='ffill')
   0  1  2
0  1  2  3
1  4  2  3
2  4  2  9