Add meta-command to artdaq for controlling CommandableFragmentGenerators
CommandableFragmentGenerator implementers may want the ability to support some arbitrary command sent in from the system. The artdaq "meta_command" command fulfills this function. Syntax is
meta_command <cmd string> <argument string>, and it is up to the implementation to decide the intepretation of the cmd and arg parameters. If the command is unsupported, the metaCommand function of CommandableFragmentGenerator should return true.