NodeSet derived classes made easy

[thiell]

Need to improve the way to derive NodeSet class in order to change parsing. As an example, we need to be able to easily implement a Lustre TargetSet class (item of the form fsname-OST0001).

As of today, there is a ParsingEngine class to deal with string parsing, but is not able to modify the way the nodeset is displayed. This needs to be improved.

After that, we need to add documentation on how to do it.