added mp.pool in _generate_samples

[astrogilda]

Pull Request Template

Description

added n_jobs (1 by default) to _generate_samples, leveraging mp.pool to parallelize generation of bootstraps.
closes #149

Type of change

Please delete options that are not relevant.

• [x ] New feature (non-breaking change which adds functionality)
• [ x] This change requires a documentation update

Checklist:

• [x ] My code follows the style guidelines of this project
• [ x] I have performed a self-review of my own code
• [ x] I have commented my code, particularly in hard-to-understand areas
• [ x] I have made corresponding changes to the documentation