collectSubscribers() and getTotalCustomers() now need a powertype. NULL
will make them work for every powertype (as before)

improvetariffs() will now use the new collectSubscribers() and
getTotalCustomers() to efficiently improve tariffs.

More importantly added a new hashmap<PowerType,
List<TariffSpecification> called ourTariffs that will hold the tariffs
that we have publish()'ed making easier to access and improve them