Calculate the number of possible combinations between baits and fragments, excluding self-ligations and only counting bait-to-bait interactions once (e.g. a-b, not b-a)
get.combination.count(baits, fragments, cis.only = FALSE)
vector of bait IDs in form chrN:start-end
vector of fragment IDs in form chrN:start-end
logical indicating whether cis-interactions only should be considered
total number of possible combinations