Merge branch 'rework_command_class' of salsa.debian.org:JRomain/diffoscope into master (Re: !63)

* 'rework_command_class' of salsa.debian.org:JRomain/diffoscope:
  Rename Difference.from_command to Difference.from_operation
  Rename command variables to more generic operation when relevant
  Cleanup operation and command attributes and methods
  Make Command subclass of new generic Operation class