Allow an analysis without providing a metadata file

[Patg13]

In some rare cases, a user could want to launch a basic analysis without any metadata (for testing purpose, or to obtain a specific information that does not require metadata), but Q2Pipe was not designed for this. The idea here would be to add a metadata flag at the start of every scripts that contains command where metadata are needed. This flag would trigger a warning for the user and bypass any commands requiring the file.