Updated qcustomplot references to v2 API, removed from 3rdparty

In this branch I have removed qcustomplot from the 3rdparty directory and added it to the cmake files as an external library. I have also updated calls to it for the version 2 API.

The implementation of the error bars was guesswork, because I couldn't find any examples of error bars in the GUI, so if somebody could test that, I would be grateful.