RawBankReadoutStatus: add assignment

• RawBankReadoutStatus has a user-defined copy constructor, but no user-defined operator= -- instead, the compiler will generate an implicit, default one

• The defaulted assignment operator is wrong, as it invokes the implicit assigment operator of KeyedObject (which should not exist!) and as a result it (also) assigns the reference count

• Replace the implicit assignment with a user-defined assignement operator. Note that it '*this' already has a key, then this assignment operator will throw an exception

• re-write some of the logic which creates RawBankReadoutStatus to avoid first creating an instance with one key, and then later trying to modify the key. Basically, delay the creation until it is known what to create...