Create a path joining utility

Created by: scarrazza

Issue by Zaharid Thursday Jan 18, 2018 at 17:08 GMT Originally opened as https://github.com/NNPDF/nnpdfcpp/issues/61

---

Just find whatever Python's os.path.join does for UNIX and copy it to an utility function. It makes no sense to be playing with the trailing '/' of every path we have everywhere.