Add a class to move responsibility for `IDrawableObjects` out of `VolumeDrawable`

## Use Case

`VolumeDrawable` currently manages both the volume and the isosurface meshes and all of the other drawable objects. This should be refactored so `VolumeDrawable` is scoped to just the volume itself.

## Solution

- [ ] Add a new class ("`VolEScene`") that can contain drawable objects and one or more `VolumeDrawable` or `IDrawableObject` objects.

## Alternatives

_Please describe any alternatives you've considered, even if you've dismissed them_