This class is an extension of the
GeneSetCollection-class that ensures that
gene/feature sets are disjoint, i.e. that each
gene/feature appear in only one set. e.g., it is used to
store cell-specific marker genes that are used in gene
expression deconvolution methods (cf. ged).
The main difference with
GeneSetCollection-class is that the
validity method additionally ensures that there is no
duplicated gene across all sets in the collection.