Fixes#21
Add support for successive user or assistant messages in `deepseek-reasoner`.
* **deepseek_reasoner.py**:
- Modify the message handling logic to allow successive messages of the same role.
- Add a check to merge successive messages of the same role into a single message.
- Update the `process_messages` function to handle the new message format.
* **README.md**:
- Add a section explaining the new message handling capability.
- Provide examples of how to format messages with successive user or assistant messages.
* **test_deepseek_reasoner.py**:
- Add tests to ensure the model correctly processes successive user or assistant messages.
- Include test cases for both interleaved and successive messages.
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