Refactor to scikit-matter

[agoscinski]

Opened PR to organize a bit the refactor.

I would merge first:

• 140

• 149

Anything else we want to reorganize except the name?

[agoscinski]

This PR is based on PR #154

• [x] Rename occurrences skcosmo -> skmatter
• [x] Rename occurrences scikit-cosmo -> scikit-matter
• [ ] Change name and link in readthedoc (will be done on merge)
• [x] pypi release with old name with a reference to the new github and/or pypi page (the PR #154 which this branch is based on)
• [x] rename github repo (will be done on merge)
• [ ] pypi and release with new name, test releases exist (official release will be done on merge)
• [x] change the about on the front page (will be done on merge)
• [x] change link in the lab-cosmo page https://github.com/lab-cosmo/#active-projects
• [ ] make 2 new releases

[rosecers]

Took a look and made a few more changes, otherwise it looks great!