title: distribute value in a one to many join pandas
body: <p>I'm new to Pandas and i'm trying to join two dataframes in a many to one merge. Instead of getting the repeated value related to the "one" key, I want to distribute it in a certain way, ie: if the "many" key is repeated 4 times i'd like to have the value asociated to the "one" key divided by 4, not repeated four times. </p>
  
  <p>So I have: </p>
  
  <pre><code>df_many = pd.DataFrame([['23040010', '230400'], ['23040020', '230400'], ['23040030', '230400'], ['23040040', '230400']], columns=['A', 'B']) 
  df_one = pd.DataFrame([['230400', 40000]], columns = ['B', 'C'])
  
  print (df_many)
  print (df_one)
  
     A       B
  0  23040010  230400
  1  23040020  230400
  2  23040030  230400
  3  23040040  230400
  
     B      C
  0  230400  40000
  
  pd.merge(df_many, df_one, how='left', on='B')
  
      A           B       C
  0   23040010    230400  40000
  1   23040020    230400  40000
  2   23040030    230400  40000
  3   23040040    230400  40000
  </code></pre>
  
  <p>what I'm looking for is:</p>
  
  <pre><code>    A           B       C
  0   23040010    230400  10000
  1   23040020    230400  10000
  2   23040030    230400  10000
  3   23040040    230400  10000
  </code></pre>
  
  <p>Any ideas? Thanks in advance!!</p>