Support writing of ChangeSets

[frafra]

libosmium supports add_changeset since 2.6.0. It would be nice to have it in pyosmium too.

[lonvia]

I presume you mean adding the function to SimpleWriter?

This need first a mutable version of osm.Changeset for this and then an implementation of the function itself.

[frafra]

Yes, I was referring to SimpleWriter.