title: How to reset indexes when aggregating multiple columns in pandas
body: <p>I have dataframe that I am trying to group by which looks like this </p>
  
  <pre><code>Cust_ID Store_ID month lst_buy_dt1  purchase_amt    
   1       20       10     2015-10-07  100
   1       20       10     2015-10-09  200
   1       20       10     2015-10-20  100
  </code></pre>
  
  <p>I need the maximum of <code>ls_buy_dt</code> and maximum or purchase amount for each <code>cust_ID</code>, <code>Store_ID</code> combination for each month in a different dataframe. Sample ouput:</p>
  
  <pre><code>Cust_ID Stored_ID month max_lst_buy_dt tot_purchase_amt
   1       20        10      2015-10-20     400
  </code></pre>
  
  <p>My code is below .  </p>
  
  <pre><code>aggregations = {
      'lst_buy_dt1': { # Get the max purchase date across all purchases in a month
      'max_lst_buy_dt': 'max',       
      },
      'purchase_amt': {     # Sum the purchases 
      'tot_purchase': 'sum',   # Find the max, call the result "max_date"
      }
  }
  
  grouped_at_Cust=metro_sales.groupby(['cust_id','store_id','month']).agg(aggregations).reset_index()
  </code></pre>
  
  <p>I am able to get the right aggregations . However the data frame contains an additional index in columns which I am not able to get rid of. Unable to show it, but here is the result from </p>
  
  <pre><code>list(grouped_at_Cust.columns.values)
  
  [('cust_id', ''),
  ('store_id', ''),
  ('month', ''),
  ('lst_buy_dt1', 'max_lst_buy_dt'),
  ('purchase_amt', 'tot_purchase')]
  </code></pre>
  
  <p>Notice the hierarchy in the last 2 columns. How to get rid of it? I just need the columns <code>max_lst_buy_dt</code> and <code>tot_purchase</code>. </p>