ECS - MultiEntity

[MakoEnergy]

Some Entities only make sense when implemented as multiple Entities. Some system will need to put into place to enable this. This could be done a few ways.

1. A Meta-Entity object, designed to store multiple Entities. Most straightforward but the least flexible.
2. Add a container that permits the storage of Entities to the Entity class. More flexible, allows a lot of recursion, but adds memory footprint to Entities that don't use this feature.
3. Add a SubEntityComponent. Adds a pointer indirection, and updating of transforms could get messy in the sub-entity isn't aware it's attached, but resolves the issues of the other two systems.

Option 3 is the most likely to be chosen at the time of this writing.

This is 9. on #134 .