title: Country by list of years datasets - how to merge by country and year when year is not a variable
body: <p>I have GDP values listed by country (rows) and list of years (column headings) in one dataset. I'm trying to combine it with another dataset where the values represent GINI. How do I merge these two massive datasets by country and year, when "year" is not a variable? (How do I manipulate each dataset so that I introduce "year" as a column and have repeating countries to represent each year?</p>
  
  <p>i.e. from the top dataframe to the bottom dataframe in the image?</p>
  
  <p><img src="https://i.stack.imgur.com/KpvGr.jpg" alt="">  </p>