Filters word combinations based on keyword density if the word count is 200 or over.

@param {WordCombination[]} wordCombinations The word combinations to filter.
@param {number} wordCount The number of words in the total text.
@param {number} lowerLimit The lower limit of keyword density.
@param {number} upperLimit The upper limit of keyword density.
@returns {WordCombination[]} Filtered word combinations.
function filterOnDensity( wordCombinations, wordCount, lowerLimit, upperLimit ) {
	return wordCombinations.filter( function( combination ) {
		return ( combination.getDensity( wordCount ) >= lowerLimit && combination.getDensity( wordCount ) < upperLimit
		);
	} );
}