Fix for joblib not saturating CPU during multiprocessing

[janko-petkovic]

What does this implement/fix? Explain your changes

This PR refactors thesimulate_for_sbi method so that now:

• it does not rely anymore on simulate_in_batches
• it provides a working solution for the multiprocessing issue explored in #1175 ...

Does this close any currently open issues?

Fixes #1175

Any relevant code examples, logs, error output, etc?

Refer to the thread in #1175 ...

Any other comments?

Further refactoring will be needed in order to avoid the process_simulator wrapping (possibly reducing the simulation runtime by roughly another 3 times) ...

Checklist

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