Explain what the Element::index field holds

Also, remind us to make the field private when that becomes possible.

app-proto/src/assembly.rs

@@ -19,7 +19,11 @@ pub struct Element {
     pub representation: DVector<f64>,
     pub constraints: BTreeSet<ConstraintKey>,
     
-    // internal properties, not reflected in any view
+    // the configuration matrix column index that was assigned to this element
+    // last time the assembly was realized
+    /* TO DO */
+    // this is public, as a kludge, because `Element` doesn't have a constructor
+    // yet. it should be made private as soon as the constructor is written
     pub index: usize
 }