Copy constructors in non-leaf classes

As discussed with Nikola yesterday: The classes inheriting from Cloneable need to explicitly make their copy constructors protected or = delete, as otherwise the compiler will implicitly generate a public copy constructor. This should be added everywhere necessary and commented appropriately (even in current leaf classes, in case someone derives from them at a later point).

The same is true with copy assignment, however that can be taken care of by = deleteing it in Cloneable. That should be ok, since we will (usually) never want to copy assign polymorphic classes anyway.

There is a special case for LinearOperator: it currently exposes public copy construction and assignment, which is required for the composite operator functionality to work, but is hugely problematic otherwise (it can result in slicing when, e.g. assigning a Scaling operator to a LinearOperator). I currently have no good idea how to solve this (and keep the value semantics of composite LinearOperators at the same time), excepting trying to restrict their usage somehow through clever protected and friend manipulations.