PR 567: makes all dense solvers inherit SoverBase (LU,Cholesky,QR,SVD).

This changeset also includes:
 * add HouseholderSequence::conjugateIf
 * define int as the StorageIndex type for all dense solvers
 * dedicated unit tests, including assertion checking
 * _check_solve_assertion(): this method can be implemented in derived solver classes to implement custom checks
 * CompleteOrthogonalDecompositions: add applyZOnTheLeftInPlace, fix scalar type in applyZAdjointOnTheLeftInPlace(), add missing assertions
 * Cholesky: add missing assertions
 * FullPivHouseholderQR: Corrected Scalar type in _solve_impl()
 * BDCSVD: Unambiguous return type for ternary operator
 * SVDBase: Corrected Scalar type in _solve_impl()

Eigen/src/SVD/BDCSVD.h

@@ -39,6 +39,7 @@ namespace internal {
 
 template<typename _MatrixType> 
 struct traits<BDCSVD<_MatrixType> >
+        : traits<_MatrixType>
 {
   typedef _MatrixType MatrixType;
 };  
@@ -1006,7 +1007,7 @@ void BDCSVD<MatrixType>::perturbCol0
 #ifdef EIGEN_BDCSVD_SANITY_CHECKS
       assert((std::isfinite)(tmp));
 #endif
-      zhat(k) = col0(k) > Literal(0) ? tmp : -tmp;
+      zhat(k) = col0(k) > Literal(0) ? RealScalar(tmp) : RealScalar(-tmp);
     }
   }
 }