Minor usability updates

[telatin]

• Updated help specifying that default --wdir is $PWD/virsorter-out/ and not $PWD (i.e. if the user specifies $PWD as --wdir the behaviour change and the output files and directories are indeed placed in $PWD)
• Added a warning if the --wdir is found: if the directory is present because created by the user or by an aborted VirSorter run, this will result in the Step 1 failing. The behaviour of the script has not changed to keep it consistent for the users, just a warning for beginners that will better understand the failure.
• Verbose mode implemented (to be expanded)

[simroux]

Thanks ! I'll clarify the help on the "Step 1 failing" issue because VirSorter behavior is not necessarily intuitive