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
@mbabker This I didn't reverted and actually I removed the second optional parameter: this is a protected function, only used inside this class, and it was always called without the second optional parameter.

If I'm wrong with this, please advise... (yes, also in a "blunt" way! :smile: )