title: Groupby column and find min and max of each group
body: <p>I have the following dataset, </p>
  
  <pre><code>        Day    Element  Data_Value
  6786    01-01   TMAX    112
  9333    01-01   TMAX    101
  9330    01-01   TMIN    60
  11049   01-01   TMIN    0
  6834    01-01   TMIN    25
  11862   01-01   TMAX    113
  1781    01-01   TMAX    115
  11042   01-01   TMAX    105
  1110    01-01   TMAX    111
  651     01-01   TMIN    44
  11350   01-01   TMIN    83
  1798    01-02   TMAX    70
  4975    01-02   TMAX    79
  12774   01-02   TMIN    0
  3977    01-02   TMIN    60
  2485    01-02   TMAX    73
  4888    01-02   TMIN    31
  11836   01-02   TMIN    26
  11368   01-02   TMAX    71
  2483    01-02   TMIN    26
  </code></pre>
  
  <p>I want to group by the Day and then find the overall min of TMIN an the max of TMAX and put these in to a data frame, so I get an output like...</p>
  
  <pre><code>Day    DayMin    DayMax
  01-01  0         115
  01-02  0         79
  </code></pre>
  
  <p>I know I need to do,</p>
  
  <pre><code>df.groupby(by='Day')
  </code></pre>
  
  <p>but I am a stuck with the next step - should create columns to store the TMAX and TMIN values?</p>