Add new ‘distanceNormalizedMostFrequentK(…)’ metric method (#8)

• Rename String extension source and test files.

• Refactor ‘distanceMostFrequentK(…)’ method

Make ‘mostFrequentKHashing(…)’ and ‘mostFrequentKSimilarity(…)’ private methods, and rename them for best pracices.

• Rename various internal variables for ‘distanceMostFrequentK’ calculations (also clamp ‘K’ to ‘sortedFrequencies.count’ instead of ‘self.count’).

• Reorganise ‘StringMetric.swift’ to put public methods in alphabetical order.

• Initialize Dictionary from tuple array using ‘reduce’ instead of a for-loop.

• Adds a new ‘distanceNormalizedMostFrequentK(between target: String, k: Int)’ method.

• Use modern Apple Quick Help syntax for method documentation comments.

• Change ‘testDistanceMostFreqK(…)’ method signature back to avoid breaking API changes.

• Improve readability of ‘mostFrequentKHash’ initialization.

• Change comment for accuracy.

• Use ‘reduce(…)’ when initializing ‘characterFrequencies’ to avoid defining it as ‘var’ and manually iterating over the characters.

• Optimize ‘mostFrequentKSimilarity(…)’ implementation.

• Update README.md