Return the value of a category used for facets computations for this
request. For a count request this would be the count for that facet, i.e.
an integer number. but for other requests this can be the result of a more
complex operation, and the result can be any double precision number.
Having this method with a general name value which is double
precision allows to have more compact API and code for handling counts and
perhaps other requests (such as for associations) very similarly, and by
the same code and API, avoiding code duplication.
arrays - provider for facet arrays in use for current computation.
ordinal - an index into the count arrays now in effect in
arrays. E.g., for ordinal number n, with
partition, of size partitionSize, now covering n,
getValueOf would be invoked with idx
being n % partitionSize.