Finished exercises 5,6
Completed all tasks for Exercises 5 and 6. All questions which are in these exercises are answered in tasks.txt file (except the last one). Also, tasks.txt contains tips about the use of my main.cc, which tests the validity of Series, ComputeArithmetic, ComputePi and RiemannIntegral. Lastly, I did some minor to PrintSeries (there is no difference) by mistake, watch out for conflicts! ...sorry :(