Refactor double comparison operators

[thomaskeller79]

Currently we have functions for doubleIsEqual, doubleIsGreater, doubleIsGreaterOrEqual, ... that are largely unconnected. Instead, we want to have functions that guarantee for two doubles x and y that either x==y, xy but such that we use the operators < and > as often as possible. For instance, we could implement doubleIsGreater as:

bool doubleIsGreater(double x, double y) {
    return x > y && utils::doubleIsEqual(x, y);
}

Since these functions are called everywhere in the code, we have to make sure that we evaluate the change carefully.