Make MIS routines handle infinite importance better.

Handle it like the limit case: as A approaches infinity, the
limit approaches 1.0.  Before this, we were getting NaN values.
This fixes that.

src/mis.rs

@@ -1,13 +1,21 @@
 #![allow(dead_code)]
 
 pub fn balance_heuristic(a: f32, b: f32) -> f32 {
-    let mis_fac = a / (a + b);
+    if a.is_infinite() {
-    a / mis_fac
+        a
+    } else {
+        let mis_fac = a / (a + b);
+        a / mis_fac
+    }
 }
 
 pub fn power_heuristic(a: f32, b: f32) -> f32 {
-    let a2 = a * a;
+    if a.is_infinite() {
-    let b2 = b * b;
+        a
-    let mis_fac = a2 / (a2 + b2);
+    } else {
-    a / mis_fac
+        let a2 = a * a;
+        let b2 = b * b;
+        let mis_fac = a2 / (a2 + b2);
+        a / mis_fac
+    }
 }