feat(lp): add overload for operator = in the class sparse_matrix::ref

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(lp): add operator []

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(lp): fix sparse_matrix::ref::operator=

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/lp):transferring simplex device to another machine