improve handeling of layer densities

- when a layer's composition isn't found in the (internal) dictionary of
  compounds, estimate its density using a weighted average of elemental
  densities.
- this fixes the annoying behaviour of having an "undefined" density passed
  to the generated input file when the user forgets to update the density
  manually.

TrimSPlib.js

@@ -581,6 +581,27 @@ function rho_fun()
         "":""
     }
     document.getElementById(rhoLi).value = rhos[chem];
+
+    // suggest a material density based on composition (when it's undefined)
+    if (!isFinite(document.getElementById(rhoLi).value)) {
+        let layer_formula = parse_formula(chem);
+
+        // determine the stoichiometry sum (for normalization)
+        let stoichiometry_sum = 0;
+	    for (key of Object.keys(layer_formula)) {
+	        stoichiometry_sum = stoichiometry_sum + layer_formula[key];
+	    }
+
+	    // determine the density using on a weighted average of the elemental densities
+	    let density_estimate = 0;
+	    for (key of Object.keys(layer_formula)) {
+	        if (layer_formula[key] != null) {
+	            density_estimate = density_estimate + (layer_formula[key] / stoichiometry_sum) * rhos[key];
+	        }
+	    }
+
+        document.getElementById(rhoLi).value = density_estimate;
+    }
 }
 
 function openTab(event, tabName) {