Get rid of DIRECTORY_SEPARATOR

This is a first tentative of standardizing on `/` as the directory separator (in Windows too...) and render the usage of DIRECTORY_SEPARATOR unneeded.

It **seems** to work but thorough testing is needed, especially under Windows.

I'm unsure of what I did with unit tests: they should be particularly screened and tested
I agree on all the lines.

Again, this must be seen as a quick-and-dirty test to check functionality.
At a second time, if everything works, all that can and all that must be "factored in" the API endpoints will be...