LIBSsa version 2.2

This is the "ready-to-use" version of LIBSsa after the editorial process of JOSS (Journal of Open Source Software). This release includes several improvements based on reviewers, including (but not limited to):

• Better README
• Contributing guidelines
• Simple tests usint pytest
• PyPI packaging (users can install with pip install libssa)
• Zenodo DOI

LIBSsa version 2.1 launch version!

This is the first version of the application made available to the general audiences, as the repository has now been made public.