Description

The description system must be used only for the `solve` function otherwise it is hard to follow if there is a default value or a priority system.

https://github.com/control-toolbox/CTDirect.jl/blob/c1a3b2d965e9f4fdd8246defb305e67c5ab0decb/src/solve.jl#L349

Replace the description by a unique Symbol for the model backend.