Add new method to set the variables for the whole parallel construct

[scastd]

Describe the feature

To avoid passing a Variables instance to every method, a new one could be created to store them for all the chained calls, as follows:

Parallel.withThreads(4)
        .withVariables(variables)
        ...
        .join();

Additional information

• [x] Would you be willing to help implement this feature?

Final checks

• [X] Read the contribution guide.
• [X] Check existing discussions and issues.