title: UITableView height of a section
body: <p>I have a UITableView that has 3 sections. My individual cells vary in height (collapse-expand).
  I need a way to figure out the height of each individual section, i.e. the sum of cell heights in each section. Preferably without calculating everything the tableView already has done each time I need it.</p>
  
  <p>I there a way to deduce or access such a value?</p>