create_sim_index: Create light-weight similarity index for fast lookups of...
In ontologySimilarity: Calculating Ontological Similarities

Create light-weight similarity index for fast lookups of between term set similarity.

create_sim_index(
  ontology,
  term_sets,
  information_content = descendants_IC(ontology),
  term_sim_method = "lin",
  combine = "average"
)

`ontology`	`ontology_index` object.
`term_sets`	List of character vectors of ontological term IDs.
`information_content`	Numeric vector of information contents of terms (named by term)
`term_sim_method`	Character string equalling either "lin" or "resnik" to use Lin or Resnik's expression for the similarity of terms.
`combine`	Character string - either "average" or "product", indicating whether to use the best-match-product' method, or function accepting two arguments - the first, the similarity matrix obtained by averaging across term sets in `term_sets`, and the second averaging across those in `term_sets2`.