title: Get the financial year from a date in a pandas dataframe and add as new column
body: <p>This must be really simple but can't find the answer - have a dataframe with a date, and i just need to add a financial year column (1st April - 31st March) - e.g. 15/5/19 would return 2019 or 2019-20.</p>
  
  <p>I could probably do it with a for loop, but i'm guessing there is a much better way within the dataframe?</p>
  
  <pre><code>if base_date.month &gt; 3:
      fin_year = base_date.year
  else:
      fin_year = base_date.year-1
  </code></pre>