Ensure consistency in resolving filepaths and sample folder paths in TQSystematicsHandler. Also some refactoring.

Previously there was only one place (albeit the most important place: where the sample folder is retrieved) where empty paths were correctly resolved to the default (nominal) ones. Now, the corrpesonding nominal path is copied when a systematic is added with an empty path. A consequence of this is that nominal paths must be set before adding systematics if the user wants to use these defaults.

Also, I did some refactoring of the areas my changes touch.