title: Filling missing values with mode of column relative to another column in Python
body: <p>I have a dataframe with 2 columnns City_Type and Vehicle_Type among many other columns. Now i have some missing values in Vehicle_Type columns which i need to impute with mode of Vehicle_Type based on City_Type. 
  First i need to write a python program to get the below output. Then i need to fill the missing values in Vehicle_Type field based on the below output.</p>
  
  <pre><code>City_Type    Vehicle_Type
  Tier1        Sedans
  Tier2        Hatchback
  Tier3        SUV
  </code></pre>