Implement standard deviation and mean calculation

Implement functions to calculate mean and standard deviation to the
Measurements class. Modify the output to print the mean into the file as
well. Make the rounding procedure in measureWallTime a separate
function to be used in these calculations. Modify the measuring
functions so that the first result is not being used (see comment).
Lastly, define a maximum value to be passed to the fibonacci
implementations to avoid overflows and calculation of wrong results.