add toc-style functionality to clouds/parcels

Can currently write to an objectRegistry, but cannot otherwise ascertain which fields a cloud would normally write.

Add a classes() method similar to what objectRegistry and IOobjectList have. However, it is probably inconvenient to have the matcher predicate.

In the cloud:

template<class CloudType>
void queryClasses(const CloudType&, HashTable<wordHashSet>& classes) const
{
    classes(IOField<label>::typeName).insert
    ({
        "active",
        "typeId",
    });

    classes(IOField<scalar>::typeName).insert
    ({
        "nParticle",
        "d",
        "dTarget",
        "rho",
        "age",
        "tTurb",
    });

    ...
}

@andy