Example

[rofinn]

Include an example in the docs which demonstrates how to use the Dataset type to perform batched / constrained operations over a collection of KeyedArrays. This example should include:

1. Construction from multiple tables
2. Filtering via multiple dimensions
3. Imputing values
4. Flattening dimensions
5. Concatenating multiple components together to produce X, y, X̂ and ŷ values