big huge changes in LU!

* continue the decomposition until a pivot is exactly zero;
  don't try to compute the rank in the decomposition itself.
* Instead, methods such as rank() use a new internal parameter
  called 'threshold' to determine which pivots are to be
  considered nonzero.
* The threshold is by default determined by defaultThreshold()
  but the user can override that by calling useThreshold(value).
* In solve/kernel/image, don't assume that the diagonal of U
  is sorted in decreasing order, because that's only approximately
  true. Additional work was needed to extract the right pivots.

Eigen/src/Core/util/Constants.h

@@ -258,6 +258,11 @@ namespace {
   EIGEN_UNUSED NoChange_t NoChange;
 }
 
+struct Default_t {};
+namespace {
+  EIGEN_UNUSED Default_t Default;
+}
+
 enum {
   IsDense         = 0,
   IsSparse        = SparseBit,