move isCutoff in rankedAbundance to computationDetails

[asizemore]

Currently, within the rankedAbundance function a boolean isCutoff gets set indicating if we are only returning the top n taxa or if we're returning all of them. This isCutoff is getting stored in the parameters slot of the result object, which is not a good place for it. Should go in computation details or elsewhere tbd.

[asizemore]

And we need to add the actual parameters to the paramters slot.