SampleConcurrent add ability to use a model

[AnthonyLloyd]

I've realised why John Hughes has a model for parallel testing. If the actual is very time or resource consuming this could mean the linearization check will be slow. Think of his examples of dropbox and a payments database.

This will need to be a major version release but steps are:

• [x] Refactor GenOperation<Actual, Model> to have 2 func one for actual and one for model.
• [x] Make MAX_CONCURRENT_OPERATIONS a parameter with a default of 5, add similar for serial operations.
• [x] Number of concurrent operations should be [2, MAX_CONCURRENT_OPERATIONS].
• [x] Consider renaming to SampleParallel (though wouldn't want to rename SampleModelBased to SampleStateful).
• [x] Consider running some serial operations before the parallel ones for both with and without model.
• [x] Add overrides to SampleConncurrent to use GenOperation<Actual, Model> and linearize the model only.
• [x] Add tests for the new SampleParallel.