title: Write multiple csv files by group
body: <p>I would like to divide my dataframe into multiple csv files by group "Rkey". For example, the sample data below would result in 3 different csv files being made, one for each unique Rkey group: R01, R02, and R03. Each of the three files would only include the rows belonging to its respective group. The names of these files can match the Rkey group name.</p>
  
  <p>How should I work this out? many thanks!</p>
  
  <pre><code>Pkey    Rkey    Var1     Var 2
  R01_1   R01  0.0000861   0.0021976 
  R01_2   R01  0.0157098   0.0415425 
  R01_3   R01  0.0142236   0.0316527 
  R01_4   R01  0.0000328   0.3496403 
  R01_5   R01  0.0122406   0.1739126
  R02_1   R02  0.0000856   0.0000915 
  R02_2   R02  0.0002946   0.0006898 
  R02_3   R02  0.0209878   0.0209901 
  R02_4   R02  0.0001359   0.0008970 
  R02_5   R02  0.0011158   0.0023558 
  R02_10  R02  0.0015220   0.0019581 
  R02_11  R02  0.0004664   0.0385724 
  R02_12  R02  0.0000095   0.3224465 
  R03_1   R03  0.0008863   0.0056300 
  R03_2   R03  0.0000021   0.0000185 
  R03_3   R03  0.0000170   0.0001655 
  </code></pre>