title: Parsing chemical formula
body: <p>Parsing: Write a function which will take a string representing a chemical species
  and return a list of tuples consisting of elements and corresponding subscripts. In
  the absence of a subscript, the subscript should be 1. Example: calling your function
  with an input of: </p>
  
  <p><code>H2SO4</code> </p>
  
  <p>should return an output of: </p>
  
  <p><code>[('H', 2), ('S', 1), ('O', 4)]</code></p>
  
  <p>So I'm trying to do a project but I'm not sure how to start it</p>
  
  <p>can anyone help me how to start this?</p>