Factored-customer model allows for multiple subscriptions by a single customer

[jecollins]

The DefaultFactoredCustomer type is designed to encapsulate a list of CapacityBundle instances, and it's the CapacityBundles that evaluate tariffs and hold TariffSubscriptions. At this point there do not appear to be any FactoredCustomer instances that take advantage of this design, but it's the CapacityBundle that holds the CustomerInfo. It is not at all clear how this feature could be used correctly. Unless someone comes up with a compelling reason to retain this feature, I recommend we remove it and either fold CapacityBundle into FactoredCustomer or make the relationship 1:1.