title: return rows in a dataframe closest to a user-defined number
body: <p>I have a user defined number which I want to compare to a certain column of a dataframe. </p>
  
  <p>I would like to return the rows of a dataframe which contain (in a certain column of df, say, df.num) the 5 closest numbers to the given number x. </p>
  
  <p>Any suggestions for the best way to do this without loops would be greatly appreciated. </p>