add partial-pivoting LU decomposition

the name 'PartialLU' is not meant to be definitive!
make inverse() and determinant() use it, so it's *almost* considered
well tested.

Eigen/src/LU/Determinant.h

@@ -51,7 +51,7 @@ template<typename Derived,
 {
   static inline typename ei_traits<Derived>::Scalar run(const Derived& m)
   {
-    return m.lu().determinant();
+    return m.partialLu().determinant();
   }
 };