Quadratic function: follow the herd?

QP solvers usually describe quadratic functions as:

```
f(x) = 0.5 x^T H x + x^T g + λ
```

but in RobOptim, `GenericNumericQuadraticFunction` considers:

```
f(x) = x^T A x + x^T b + c
```

Thus, we have `H = 2 A`, which may be a source of error for users used to traditional QP solvers.
