Feature request: concatenate chains from multiple directories into single Core

[AaronDJohnson]

Pat Meyers requested that we add a feature to the Core objects to easily combine multiple independent chains into a single Core object.

This could work by allowing the user to input a list of chain directories into Core at which point they would have the burn-in segments on each chain removed and the chains concatenated.

This could also be extended to HypermodelCores (I think SlicesCore already does this for its use cases).