CMake corrections

fix for issue 2510 "GUI: Unreadable text in error dialogs"

Examples: get_simulation now uses c'tor GISASSimulation(beam, sample, det). plot_utils: simplified handling of keyword args.

GUIHelpers::Error::what() returned a char* to a temporary QByteArray which is invalid after leaving what()

Fixes the error that in a second run of CMake (meaning with a CMakeCache.txt present) the var Cerf_IS_CPP was always set to OFF

because of integrator failure.