improve way to instace object 2d

[Juanperias]

The way in which the struct Object2d is instantiated can be cryptic, e.g.:

let mut obj = obj2d(
    vec2(75.0, 200.0),
    2.0,
    2.0,
    vec2(2.0, 231.0),
    vec2(2.0, -240.0),
    20.0,
    Shape::Circle,
);

This pull request sets obj2d as deprecated and add this way:

    let obj = Object2dBuilder::new()
        .position(vec2(2.0, 2.0))
        .density(2.0)
        .mass(3.0)
        .velocity(vec2(4.0, 4.0))
        .acceleration(vec2(3.0, 3.0))
        .radius(2,0)
        .shape(Shape::Circle)
        .build();

```