Replaced `BatchedExecutor.Prompt(string)` method with `BatchedExecutor.Create()` method. This improves the API in two ways:
 - A conversation can be created, without immediately prompting it
 - Other prompting overloads (e.g. prompt with token list) can be used without duplicating all the overloads onto `BatchedExecutor`

Added `BatchSize` property to `LLamaContext`

Modified LLamaBatch to not share tokens with other sequences if logits is true. This ensures that the logit span at the end in used by exactly one sequence - therefore it's safe to mutate. This removes the need for copying _very_ large arrays (vocab size) and simplifies sampling pipelines.

* Added a `Guidance` method to `LLamaTokenDataArray` which applies classifier free guidance

* Factored out a safer `llama_sample_apply_guidance` method based on spans

* Created a guided sampling demo using the batched executor

* fixed comment, "classifier free" not "context free"

* Rebased onto master and fixed breakage due to changes in `BaseSamplingPipeline`

* Asking user for guidance weight

* Progress bar in batched fork demo

* Improved fork example (using tree display)

* Added proper disposal of resources in batched examples

* Added some more comments in BatchedExecutorGuidance