This interface must be implemented by any class used as file-diff facility in assertFile functions.
It declares two functions, which are called whenever the file comparision is required. Their meaning
is identical, they only differ by arguments types.
Generally, they both take three parameters, the first two specify files being compared and the third
is the file, where comparision results are stored. Third paramtere can be null in case no additional
output except the return value is needed.