title: pandas create a new column with condition (max date and == condition on existing columns)
body: <p>I have a dataframe with following data:</p>
  
  <pre><code>id|invoice_no|invoice_date|change_previous_month|change_status
  
  984974|110|2016-12-31|0|A
  
  984974|8202|2017-01-30|-64864|D
  
  115677|5505|2016-12-31|0|A
  
  115677|5635|2017-01-30|58730|U
  </code></pre>
  
  <p>i want to create a new column 'event_date' from 'invoice_data' based on conditions on existing columns. 
  the conditions are:</p>
  
  <p>1) select maximum 'invoice_date' that is defined as the latest date and </p>
  
  <p>2) select 'change_status' where 'change_status' == 'A' or 'change_status' == 'U'</p>
  
  <p>The resulted dataframe should look like this:</p>
  
  <pre><code>id|invoice_no|invoice_date|change_previous_month|change_status|event_date
  
  984974|110|2016-12-31|0|A|2016-12-31
  
  984974|8202|2017-01-30|-64864|D|2016-12-31
  
  115677|5505|2016-11-31|0|A|2017-01-30
  
  115677|5635|2017-01-30|58730|U|2017-01-30
  </code></pre>
  
  <p>the event_date should be created from the invoice_date with satisfying the above two conditions ,thanks in advance for your help.</p>