First release

Summary

Changes compared to https://github.com/broadinstitute/ssGSEA2.0:

• Clean up dependencies
• Convert to R package ssGSEA2
• Rename ssGSEA2 to run_ssGSEA2 to avoid documentation conflicts with package name
• Separate functions and add doc skeletons
• Add output.directory argument and write all outputs there instead of PWD
• In accordance with Bioconductor standards, replace '.' with '_' in function names
• Add example

Full Changelog: https://github.com/nicolerg/ssGSEA2/commits/v1.0.0