Physics library for use in Specs, using cgmath and collision-rs.

# Example

```rust
extern crate cgmath;
extern crate rhusics;
extern crate shrev;
extern crate specs;

use cgmath::{Point2, Rad, Rotation2, Transform};
use shrev::EventChannel;
use specs::{RunNow, World};

use rhusics::ecs::collide::prelude2d::{register_collision, BasicCollisionSystem2, BodyPose2,
                                       BroadBruteForce2, CollisionMode, CollisionShape2,
                                       CollisionStrategy, ContactEvent2, GJK2, Rectangle};

pub fn main() {
    let mut world = World::new();
    register_collision::<f32, BodyPose2<f32>, ()>(&mut world);

    let mut reader_1 = world
        .write_resource::<EventChannel<ContactEvent2<f32>>>()
        .register_reader();

    world
        .create_entity()
        .with(CollisionShape2::<f32, BodyPose2<f32>, ()>::new_simple(
            CollisionStrategy::FullResolution,
            CollisionMode::Discrete,
            Rectangle::new(10., 10.).into(),
        ))
        .with(BodyPose2::<f32>::one());

    world
        .create_entity()
        .with(CollisionShape2::<f32, BodyPose2<f32>, ()>::new_simple(
            CollisionStrategy::FullResolution,
            CollisionMode::Discrete,
            Rectangle::new(10., 10.).into(),
        ))
        .with(BodyPose2::new(
            Point2::new(3., 2.),
            Rotation2::from_angle(Rad(0.)),
        ));

    let mut system = BasicCollisionSystem2::<f32, BodyPose2<f32>, ()>::new()
        .with_broad_phase(BroadBruteForce2::default())
        .with_narrow_phase(GJK2::new());
    system.run_now(&world.res);
    println!(
        "Contacts: {:?}",
        world
            .read_resource::<EventChannel<ContactEvent2<f32>>>()
            .read(&mut reader_1)
            .collect::<Vec<_>>()
    );
}
```

# Features:

* Has support for all primitives in collision-rs
* Has support for the following broad phase algorithms in collision-rs:
  * Brute force
  * Sweep and Prune
* Narrow phase collision detection using GJK, and optionally EPA for full contact information
* [`specs::System`](https://docs.rs/specs/0.9.5/specs/trait.System.html) for collision
  detection working on user supplied transform, and shape components.
  Can optionally use broad and/or narrow phase detection.
  Library supplies a transform implementation for convenience.
* [`specs::System`](https://docs.rs/specs/0.9.5/specs/trait.System.html) for spatial
  sorting on user supplied transform, and shape components.
* Has support for doing spatial sort/collision detection using the collision-rs DBVT.
* Support for doing broad phase using the collision-rs DBVT.
* Continuous collision detection, using GJK
* Simple rigid body implementation with single contact forward resolution

# TODO:

* Impulse solver
* Integrator implementations (Euler, RK4, etc.) 
* Parallel solver implementation