Adapted the return statement to fill a dictionary with the results and removed the print statements.

Added documentation and an accuracy function able to calculate the accuracy of a given aligned shadowing task.

Added the form mistake type to the mistake enum for good measure. Added a mistake_counter class that gets the aligned source and shadow words and outputs the number of each mistake made in the shadow task. For now also prints these values to the console, for testing purposes. Added instance of this class to the statistics class, for analysis after alignment.