Change how parser is used

[albireox]

Right now the user needs to import from clu import command_parser and append commands to it. That's not very clear. While keeping that option, it would be better to allow the user to send their own command group (as long as it uses CluGroup).

It would need to:

• If the actor receives an empty parser keyword, use the internal command_parser.
• If it's not empty, check that it uses CluGroup.
• See if the new command group implements ping, help, and other commands that are normally defined by CLU. If they are implemented, do nothing, otherwise add them from the CLU command parser.