title: Find max values of a column in a time series dataset
body: <p>I have a time series dataset <code>DF</code> where the first column is the <code>timestep</code> and the second column is the <code>cellNo.</code>. How can I drop all rows except the <code>max(DF$cellno.)</code> of <strong>each</strong> <code>timestep</code>?</p>
  
  <pre><code>&gt; head(DF, n=100)
      timestep cellNo.
  1          1       1
  2          1       2
  3          1       3
  4          1       4
  5          1       5
  6          1       6
  7          1       7
  8          1       8
  9          1       9
  10         1      10
  11         1      11
  12         1      12
  13         1      13
  14         1      14
  15         1      15
  16         1      16
  17         1      17
  18         1      18
  19         1      19
  20         1      20
  21         1      21
  22         1      22
  23         1      23
  24         1      24
  25         1      25
  26         1      26
  27         1      27
  28         1      28
  29         1      29
  30         1      30
  31         1      31
  32         1      32
  33         2       1
  34         2       2
  35         2       3
  36         2       4
  37         2       5
  38         2       6
  39         2       7
  40         2       8
  41         2       9
  42         2      10
  43         2      11
  44         2      12
  45         2      13
  46         2      14
  47         2      15
  48         2      16
  49         2      17
  50         2      18
  51         2      19
  52         2      20
  53         2      21
  54         2      22
  55         2      23
  56         2      24
  57         2      25
  58         2      26
  59         2      27
  60         2      28
  61         2      29
  62         2      30
  63         2      31
  64         2      32
  65         3       1
  66         3       2
  67         3       3
  68         3       4
  69         3       5
  70         3       6
  71         3       7
  72         3       8
  73         3       9
  74         3      10
  75         3      11
  76         3      12
  77         3      13
  78         3      14
  79         3      15
  80         3      16
  81         3      17
  82         3      18
  83         3      19
  84         3      20
  85         3      21
  86         3      22
  87         3      23
  88         3      24
  89         3      25
  90         3      26
  91         3      27
  92         3      28
  93         3      29
  94         3      30
  95         3      31
  96         3      32
  97         4       1
  98         4       2
  99         4       3
  100        4       4
  </code></pre>