Implement a new model svgp in JAX

[PatrickOHara]

@defaultobject has a repo called cleanair_aq_example that trains a JAX model on the LAQN data (see m_stgp.py).
We want to include the model into the cleanair repo.

• Make sure importing the model from the stgp repo works
• Create a new class that extends the ModelMixin class
• Implement the 'fit' method
• Implement the 'predict' method
• make sure new class works with the other classes

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