Add coverage for optimisation.parallel()

[BradyPlanden]

Feature description

Currently, optimisation.parallel() enables Pints.ParallelEvaluator for multi-threaded evaluations of the cost function vector object. Additional changes may be required for the different multi-processing context between Windows/MacOS (spawn) and Linux (fork).

At the same time, an evaluation of the performance of the parallel evaluation should be completed and an example notebook should be created.

Motivation

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Possible implementation

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Additional context

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