Fix BOLFI parallelisation

[uremes]

Summary:

copy kernel from gpy regression model gp_params to avoid the pickle issue reported here and allow BOLFI parallelisation also when a non-default kernel is used. fixes #263

How to test:

the pickle error is observed when we sample from a target_model that has a kernel in gp_params and either ipyparallel or multiprocessing client is used. for example the example code provided in #263 results in an error with the current ELFI version but works with the updated version.

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