Cleanup code + minor bug fixes

[fraimondo]

We are currently working on a research project that uses neuroHarmonize. By digging into it, we found that some issues with the code that we address in this PR:

Summary of changes:

• Fix unused imports.
• Fix bug with setting random seed in harmonizationLearn.
• Fix logic to avoid warnings due to unbound variables.
• Format code to match black.
• Add nibabel as an optional dependency.
• Migrate to pyproject.toml.
• Update installation instructions.
• Update package build and publish instructions.

[rpomponio]

Hi @fraimondo,

I appreciate you submitting this pull request. It may take me some time to review these commits. Some colleagues of mine have been working in-parallel on other commits (#28), so I'll have to resolve conflicts between the two branches.

I hope you have found this package helpful.