Consider not outputting cliques of size 2 or less

[matentzn]

Right now, on a large run we output all cliques, including those of with just two members. These are neither useful for visualisation nor for human review?

[balhoff]

@matentzn let's come back to this after looking at the latest JSON output, which includes sibling mappings rather than dropping them. I think groups of two members are meant to be basically what's in the SINGLETONS list.