moved the particle system and the entity component system into their own nuget packages

Source/MonoGame.Extended.Particles/Modifiers/DragModifier.cs

@@ -0,0 +1,23 @@
+using Microsoft.Xna.Framework;
+
+namespace MonoGame.Extended.Particles.Modifiers
+{
+    public class DragModifier : IModifier
+    {
+        public float DragCoefficient { get; set; } = 0.47f;
+        public float Density { get; set; } = .5f;
+
+        public unsafe void Update(float elapsedSeconds, ParticleBuffer.ParticleIterator iterator)
+        {
+            while (iterator.HasNext)
+            {
+                var particle = iterator.Next();
+                var drag = -DragCoefficient*Density*particle->Mass*elapsedSeconds;
+
+                particle->Velocity = new Vector2(
+                    particle->Velocity.X + particle->Velocity.X*drag,
+                    particle->Velocity.Y + particle->Velocity.Y*drag);
+            }
+        }
+    }
+}