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 {
+    if a.is_infinite() {
+        a
+    } else {
         let mis_fac = a / (a + b);
         a / mis_fac
     }
+}
 
 pub fn power_heuristic(a: f32, b: f32) -> f32 {
+    if a.is_infinite() {
+        a
+    } else {
         let a2 = a * a;
         let b2 = b * b;
         let mis_fac = a2 / (a2 + b2);
         a / mis_fac
     }
+}