When Apple introduced the latest operating system El Capitan OSX 10.11 it came with the new Photos App which seemed to lack some of the features of the earlier iPhoto app and looked pretty much like a copy of the Photos app on iOS devices.
However it appears that Photos on OSX now supports the installation of Extensions which permits third party developers to enhance the basic editing capabilities by delivering apps through the Mac App store.
There is a useful app available for $0.99 which enables external editing in Photoshop and Photoshop Elements, with the edited photo saved back automatically into the Photo Library.
External Editors For Photos by Karsten Bruns
For those who are accustomed to round-tripping from iPhoto into Photoshop, Pixelmator, Acorn, or other image editors this app which runs via Extensions could be the answer.
The developer has a short video tutorial which demonstrates how the extension works with external editors.