support creation of Structure objects single-sample (haploid) outgroup

egglib.struct_from_label() now supports (or will support soon) haploid outgroups (ougroups with a single sequence, or sample, although the ingroup has a ploidy of 2 or more).

Other ways to create structure object (such as struct_from_samplesizes(), and probably the others as well) should be extended to support this feature.