Add support for GPT-4-0806 model in models.py

[mentatai[bot]]

Objective

Add support for the new GPT-4-0806 model in the models.py file, including its updated pricing.

Background

OpenAI has released a new version of GPT-4, known as GPT-4-0806, which offers improved performance and lower prices. We need to update our spice/models.py file to include this new model option.

Implementation Steps

1. Update the spice/models.py file:
   • Add a new TextModel instance for GPT-4-0806.
   • Update the pricing information for this new model.
2. Ensure that the new model is properly registered in the models list.
3. Update any relevant documentation or README files to mention the new model option.
4. Add tests to ensure the new model can be selected and used correctly.

Code Changes

• In spice/models.py, added the new GPT-4-0806 model.
• Updated README.md to include the new model in the model_aliases.
• Added unit tests in tests/test_models.py to verify the new model's attributes and retrieval.

Testing

• Verified that the GPT-4-0806 model can be retrieved using get_model_from_name("gpt-4-0806").
• Confirmed that the model has the correct attributes (name, provider, input_cost, output_cost, context_length).

Documentation

Updated relevant documentation, such as README.md, to include information about the new GPT-4-0806 model option.

Closes #97

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