Refactoring of the cost model:

- Dynamic is now an invalid value
 - introduce a HugeCost constant to be used for runtime-cost values or arbitrarily huge cost
 - add sanity checks for cost values: must be >=0 and not too large
This change provides several benefits:
 - it fixes shortcoming is some cost computation where the Dynamic case was not properly handled.
 - it simplifies cost computation logic, and should avoid future similar shortcomings.
 - it allows to distinguish between different level of dynamic/huge/infinite cost
 - it should enable further simplifications in the computation of costs (save compilation time)

test/redux.cpp

@@ -74,7 +74,6 @@ template<typename MatrixType> void matrixRedux(const MatrixType& m)
   Matrix<Scalar, MatrixType::RowsAtCompileTime, MatrixType::RowsAtCompileTime> m2(rows,rows);
   m2.setRandom();
   VERIFY_EVALUATION_COUNT( ((m1.matrix()*m1.matrix().transpose())+m2).sum(), (MatrixType::SizeAtCompileTime==Dynamic ? 1 : 0) );
-
 }
 
 template<typename VectorType> void vectorRedux(const VectorType& w)