calculating cumulative frequency corrected

Summary:
The current implementation of the cumulative frequency is implemented very inefficient. I changed it making its runtime linear.

Additionally, I changed the method for changing an element inside a QVector. At least on my raspberry, this seems to have side effects and changed the "channels" attribute leading to a crash.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D14149