TODO

[geoffroyleconte]

1. Find a way to implement lmul! and rmul! for special operators such as OpDiagonal (maybe create separate LinearOperator types, or create a SpecialLinearOperator type that has lmul! and rmul! in its definition)
2. 5-args mul! with LBFGS (only 3-args mul! is implemented for now)
3. Operations with LBFGS operators such as + , *.
4. Flip expressions (see test_callable.jl) is not very convenient
5. kron, opInverse, opLDL... are not in-place
6. We have to write y = - (op * x) instead of y = - op * x if we use LBFGS or LSR1 operators (linked to 3. )