Add partial pivoting runtime option to LU.

Note: in fact, inverse() always uses partial pivoting because the algo
currently used doesn't make sense with complete pivoting. No num
stability issue so far even with size 200x200. If there is any problem
we can of course reimplement inverse on top of LU.

Eigen/src/Core/util/Constants.h

@@ -209,6 +209,11 @@ enum {
   HasDirectAccess = DirectAccessBit
 };
 
+enum {
+  PartialPivoting,
+  CompletePivoting
+};
+
 const int FullyCoherentAccessPattern  = 0x1;
 const int InnerCoherentAccessPattern  = 0x2 | FullyCoherentAccessPattern;
 const int OuterCoherentAccessPattern  = 0x4 | InnerCoherentAccessPattern;