Add propeller spin direction

src/integration.zig

@@ -1,6 +1,7 @@
 const drone = @import("./drone.zig");
 const vec3 = @import("./vec3.zig");
 const forces = @import("./forces.zig");
+const constants = @import("constants.zig");
 const std = @import("std");
 
 pub fn timestep(dt: f64, d: drone.Drone, props: []const drone.Propeller, s: *drone.State) void {
@@ -62,6 +63,7 @@ test "Validate timesteps" {
         .cg_to_prop = .init(1, 0, 0),
         .moment_coefficient = 0.1,
         .thrust_coefficient = 0.5,
+        .direction = constants.PropSpinDirection.cw,
     };
     const props: [1]drone.Propeller = .{prop};