use `src` layout

[d-v-b]

we have the source code for the top-level of the project in eopf_geozarr, which means running python code in the root of the project directory can import content from source files, which may not be desired when running tests, where the goal is the import code from the built version of the source code. Putting the source code inside src/eopf_geozarr would address this. see https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/

[emmanuelmathot]

fixed with #13