Bug solving in obj file export: material path as a relative path

[jdelanoy]

PR Description

This solves a bug in the export of OBJ meshes: the material link was written in aboslute path in the .obj file instead of relative path. This is solved by removing all the directory structure when writing the material path in the obj (will be written in the same directory).

Checklist

• Unit-test of your feature with Catch.
• Doxygen documentation of the code completed (classes, methods, types, members...)
• Documentation module page added or updated.
• New entry in the ChangeLog.md added.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).
• All continuous integration tests pass (Travis & appveyor)

[JacquesOlivierLachaud]

Seems fine. Do you mind updating the ChangeLog file (at the root of DGtal) ?

[dcoeurjo]

• new line in the AUTHORS file :)

[dcoeurjo]

👌🎉

[dcoeurjo]

thanks @jdelanoy ! Merging.