Improve logic in GlobalChi2Fitter

This PR makes the GX2F more readable by improving some logic:

• use else if for mutual exclusive if-chains
• consistently use && and || over and and or
• use curly braces { } also for single expression ifs
• use ternary assignment over int a = 0; if (b > 0) {a = b;}
• avoid possible implicit conversion to bool when checking for nullptr
• simplify nested ifs
• avoid negations