#38 split subsets by scaffolds on a specified level

[Till Schäfer]

create an option to split a subset by the scaffolds on a specified ring level x.

for each scaffold on level x: all molecules which belong to the scaffold (complete subtree), should be saved in a subset.

molecules from higher levels should be ignored.

[Nils Kriege]

Implemented by r2198, original patch submitted by Shamshad Alam.