title: Linearly interpolate missing rows in pandas dataframe
body: <p>I have the foll. dataframe:</p>
  
  <pre><code>               Value         ts  year   JD          check     group_id
  0                NaN  950832000  2000   49           NaN      19987
  1                NaN  952214400  2000   65           NaN      19987
  2                NaN  953596800  2000   81           NaN      19987
  3        0.402539365  954979200  2000   97           NaN      19987
  4         0.54021746  956361600  2000  113           NaN      19987
  5          0.6156816  957744000  2000  129           NaN      19987
  6         0.69301238  959126400  2000  145           NaN      19987
  7        0.745426085  960508800  2000  161           NaN      19987
  8         0.77601947  961891200  2000  177           NaN      19987
  9         0.75455118  963273600  2000  193           NaN      19987
  10        0.71251396  964656000  2000  209             1      19987
  11        0.67047674  966038400  2000  225           NaN      19987
  12        0.68953113  967420800  2000  241           NaN      19987
  13        0.63757508  968803200  2000  257           NaN      19987
  14        0.56996103  970185600  2000  273           NaN      19987
  15         0.5095817  971568000  2000  289           NaN      19987
  16        0.41956058  972950400  2000  305             1      19987
  17        0.32953946  974332800  2000  321           NaN      19987
  18  0.33679403444444  975715200  2000  337             1      19987
  19  0.34404860888889  977097600  2000  353             1      19987
  </code></pre>
  
  <p>Here, JD refers to the day of the year (Jan 1 is day 1). I would like to create a new dataframe where all the elements in the 'Value' column are linearly interpolated for all days in the year.</p>
  
  <p>I am doing this:</p>
  
  <pre><code>from scipy.interpolate import interp1d
  interp1d(df['Value'], df.index, kind='linear')
  </code></pre>
  
  <p>But this will only fill in existing values that are NaN. How do I fill in the values which are completely missing e.g. row 1 is for 49th day of year but the 50th day of year row is missing.</p>