various work on the Sparse module:

* added some glue to Eigen/Core (SparseBit, ei_eval, Matrix)
* add two new sparse matrix types:
   HashMatrix: based on std::map (for random writes)
   LinkedVectorMatrix: array of linked vectors
   (for outer coherent writes, e.g. to transpose a matrix)
* add a SparseSetter class to easily set/update any kind of matrices, e.g.:
   { SparseSetter<MatrixType,RandomAccessPattern> wrapper(mymatrix);
     for (...) wrapper->coeffRef(rand(),rand()) = rand(); }
* automatic shallow copy for RValue
* and a lot of mess !
plus:
* remove the remaining ArrayBit related stuff
* don't use alloca in product for very large memory allocation

Eigen/src/Core/Product.h

@@ -258,7 +258,6 @@ template<typename OtherDerived>
 inline const typename ProductReturnType<Derived,OtherDerived>::Type
 MatrixBase<Derived>::operator*(const MatrixBase<OtherDerived> &other) const
 {
-  assert( (Derived::Flags&ArrayBit) == (OtherDerived::Flags&ArrayBit) );
   return typename ProductReturnType<Derived,OtherDerived>::Type(derived(), other.derived());
 }