Proper `BatchedObservations` with optional weights?

Sometimes we'd like to emphasize some `SyntheticObservations` over others. We can express this by introducing an object `BatchedObservations` rather than using a simple `Vector`:

```julia
struct BatchedObservations
batch
weights
end
```

By default the `weights` are all `1`, but this could be overridden to emphasize / demphasize certain members of the batch.

Start by locating the existing SyntheticObservations representation and the code paths that consume batches. Determine how a BatchedObservations object with default or custom weights should fit those paths, and consider the behavior that would demonstrate support for emphasized and de-emphasized observations.

Written by the indexing model from the issue text.