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/Inverse.h

@@ -170,8 +170,7 @@ struct ei_compute_inverse
 {
   static inline void run(const MatrixType& matrix, MatrixType* result)
   {
-    LU<MatrixType> lu(matrix);
-    lu.computeInverse(result);
+    matrix.partialLu().computeInverse(result);
   }
 };