Improved context window size management

Context window size is largely manual right now – it can be specified via `{"options": {"num_ctx": 32768}}` in the API or via `PARAMETER num_ctx 32768` in the Modelfile. Otherwise the default value is set to `2048` unless specified (some models in the [library](https://ollama.ai/ will use a larger context window size by default) Context size should be determined dynamically at runtime based on the amount of memory available.