Provide Ref<SparseMatrix>/Ref<const SparseMatrix>

Submitted by Christoph Hertzberg @chhtz

Assigned to Nobody

Link to original bugzilla bug (#910)
Version: 3.3 (current stable)

Description

Storing references to sparse matrices is not directly possible at the moment but it would help solving Bug #897 (closed).
A question is whether Block<SparseMatrix> objects shall be storeable into a Ref object.

Similar to Ref<const Matrix<...> >, a Ref<const SparseMatrix> can hold a temporary if the passed matrix is not storage compatible with the Ref object.
Similar to Bug #884, we can think about a Ref<const SparseMatrix&> variant.

Blocking

#897 (closed)