Enable quantifier unfolding by default
Until #3703 is resolved, quantifications over subsets are only evaluated correctly if they are unfolded, i.e. explicitly
qtfer.setUnfold(true) is invoked.
To implement a more defensive strategy, the default value of
IQuantifierExpression::unfold should be set to
true and all explicit invocations of
setUnfold() in the code should be removed.