Add CI using GithubActions

[louisguitton]

Why

While contributing my last PR, I noticed we were not exercising the pre-commit hooks and the pytest tests on each branch.

This is important to add in order to lower the bar for external contributions and lower the maintenance burden.

What

Add a Github Actions Continuous Integration (CI) workflow that will:

• pip install the package (make sure install works)
• run pre-commit hooks
• run tests

[ceteri]

Many thanks!