Target Scala 2.13 and provide backward compatibiliy code

[gaeljw]

The Scala source code used for the three Scala versions (2.11, 2.12, 2.13) is the same.

While this works fine, this brings some warnings in the Scala 2.13 build as well as boilerplate code.

We should consider targeting Scala 2.13 as the main Scala version and provide some backward compatibility code for earlier versions.

The first step will be to identify the extent of this change.

Optionally but it would preferable to avoid it, we could also have 3 whole different codebase but it would be a pain to maintain.