Moving more into SystemsRx

Summary

Some of these changes actually happened in previous releases but a few things have been moved down into SystemsRx such as the pooling mechanisms, as they are useful regardless of the ECS aspect of things, so now they can be used without.

Also there have been some changes thanks to @Fijo which makes it easier to include IObservableGroup objects on the fly in your systems via attributes by using the GroupBinding plugin.

GroupBinding Plugin

This is optional but if you do want to include multiple IObservableGroup objects within your systems (such as IManualSystem) you can do so like this:

[FromComponents(typeof (LevelComponent))] public IObservableGroup LevelAccessor;     
[FromComponents(typeof(EnemyComponent))]  public IObservableGroup EnemyAccessor;

Which is directly lifted from the Roguelike2d Repository which has some real life usages of this plugin and how it can simplify the system and reduce dev time/boilerplate, there are some other approaches where you can use FromGroup to pass in an explicit group or just use FromGroup without a group on systems which implement IGroupedSystem, and it will take the default group property as its argument.

This also simplifies those usecases for people who were originally using IManualSystems before they got moved to SystemsRx and were left without a simple way to resolve an IObservableGroup when needed.