A new way for managing seeds with the --seeds option in buildgraph
In relation with issue #27 (closed), the --seeds option have to change its behavior.
If the --seeds <SEEDNAME> option is added to the buildgraph command, the new seed set is added to the seed set collection and put the new seeds in the stack for the first graph extension step.
if the --seeds <SEEDNAME> is used in conjunction with the --clean option (#27 (closed)) , all the previous seed sets are erased