add codecov yaml file
add codecov yaml file

Relevant option is to allow CI to pass, if coverage reduces by 5%. It is
quite radcial, and the exact value is to be discussed.

DiCarloLab-Delft/PycQED_py3,DiCarloLab-Delft/PycQED_py3,DiCarloLab-Delft/PycQED_py3