Add option to not add Assimulo solver object to PysMod

[c-barry]

Add option not to add cvode solver class to model after simulation as cython object prevents serialization

[jmrohwer]

I have changed the topic to a more descriptive one.

The main purpose of this is to allow the use of CVODE in parallelized scenarios, e.g. for doing parameter identifiability analysis. See e.g. https://github.com/jmrohwer/identifiability

@bgoli any objections to this?

[bgoli]

thanks for the patch