GitHub actions v1

[tristan0x]

This contribution adds the first brick of CI of this project by checking:

• the formatting of both CMake and Python code
• the static analysis (with flake8) of the Python code

---

blocked by #123

[tristan0x]

Looks reasonable to me; I had a couple of comments and I don't think .ci/.check_cmake_format.sh.swp is meant to be committed?

oops :)