Issues with jax usage

[kaiserls]

• Document classmethod solution for pickling function, how to use fixed params
• But also investigate why jitting the temperature model generates super slow code while super fast for e. g. corona. jitted should be not slower than normal python code.
• Use them in tutorial / provide tutorial.py
• Improve user friendliness of jax models: https://stackoverflow.com/questions/1816958/cant-pickle-type-instancemethod-when-using-multiprocessing-pool-map/41959862#41959862

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[kaiserls]

I just found a jax port of emcee: dfm/emcee#499