Close #43

This takes rendering time from ~0.5ms to ~0.5ms.
When highlighting a range it may get up to 1.5ms.

src/app/logic/calc.rs

@@ -489,6 +489,8 @@ impl Grid {
         }
     }
 
+    /// Gets the indices of the range labels.
+    /// A:B -> (0,1)
     pub fn range_as_indices(range: &str) -> Option<(usize, usize)> {
         let v = range.split(':').collect::<Vec<&str>>();
         if v.len() == 2 {