extraction operations

As just discussed with @Mattijs, there are several places where we wish to do a list reduction of particular object quantities. For example, to obtain a list of names/types/sizes from the objects. The ListOps::create function will do some of that, but would benefit from some predefined functors.

Eg,

template<class T> struct nameOp
{
    word operator()(const T& obj) const { return obj.name(); }
};

And later

ListOps::create<word>(patches, nameOp<polyPatch>());

Or

ListOps::create<word>(readDir(dirName), nameOp<fileName>());