Abstract: future

@ocots @PierreMartinon @joseph-gergaud @j-l-s Some todo's suggestions:
- [x] add single sided inequalities (see also control-toolbox/CTBase.jl#67 as having a kwargs version of `constraint!` does help for this)
- [x] add reverse inequalities (≥)
- [ ] declare time, state, control, variable simultaneously according to
```julia
(t, x, u) ∈ [ 0, 1 ] × R³ × R²
(t, x, u, v) ∈ [ 0, v₂ ] × R³ × R² × R² # meaning that final time is free and equal to v[2] = v₂
```
- ~~[ ] `in place` dynamics (`f!(ẋ(t), x(t), u(t))`) or even~~ DAE form (`F(ẋ(t), x(t), u(t)) == 0`)
- ~~[ ] for the `in place` form, using a keyword like~~
```julia
ẋ(t) == f(x(t), u(t)), inplace
```
- [ ] parse dynamics in several steps: `x'[2](t) == ...`, ~~`x'[2:4](t) == ...`~~ (each time call `dynamics!`, and assemble afterwards what the dynamics is...) https://github.com/control-toolbox/CTParser.jl/issues/22
- [ ] allow `@Lie [ X, Y ]` and `@Poisson { f, g }` directly (= without `@`) in the DSL?