Check that path provided is absolute for 'dataset format' command

I'm raising this issue after a bit of confusion occured when dicussing how teklia-dan dataset format works with Solene,

According to the documentation, the --dataset parameter is meant to be used like an absolute path, but the command works even if a relative path is given. For instance, using --dataset = data, since teklia-dan dataset extract creates such a directory.

This causes the paths in labels.json to be relative, which is counterintuitive and can cause some issues later.

I propose to automatically convert relative paths to absolute if using this command, if possible.