Updated registration process in domain app

The registration process in the domain app was updated with the
following logic:
- Registration commands for non-existent charging stations are now
  handled like boot commands. It is checked whether or not the charging
  station already exists. If it does, the registration command is
  handled as usual. If it doesn't, the command handler first creates the
  charging station aggregate.
- Registering an already registered charging station now throws an
  IllegalArgumentException.