and (more importantly) avoid evaluating column pairs more than once

are explicitly passed in. Also now testing all column pairs, even for columns
that have already been flagged as redundant. May revert this though. New
util.dedup function.

Enh/index visits

Closes #8 and #11

See merge request !31

refactorings to remove assumptions of single-column index

deprecated lowMemory/workDir arguments, because I'm pretty sure nobody's
using them.

on import - not sure why it was happening here? Perhaps it was once upon a
time

not doing a big rum

Rf/remove subjects parallel

Closes #9

See merge request !30