- optimized determinant calculations for small matrices (size <= 4)

(only 30 muls for size 4)
- rework the matrix inversion: now using cofactor technique for size<=3,
  so the ugly unrolling is only used for size 4 anymore, and even there
  I'm looking to get rid of it.

Eigen/LU

@@ -5,6 +5,7 @@
 
 namespace Eigen {
 
+#include "Eigen/src/LU/Determinant.h"
 #include "Eigen/src/LU/Inverse.h"
 
 } // namespace Eigen