Scale/UpDown operators rely on StateNode.scale() -- replace by Scalable interface

[rbouckaert]

StateNode has a scale method that is not implemented in all StateNodes since it does not make sense to scale an Alignment, or BooleanParameter for example. Even IntegerParameter does not properly scale.

Introducing a Scalable interface that allows appropriate StateNodes to implement scale and for others to ignore it means we can feed only Scalable objects to ScaleOperator and UpDownOperator. Perhaps amalgamating the scale and up-down operator now it only applies to Scalable state nodes would be nice. It also means we don't have to distinguish between Tree, RealScalar and RealVector scale operators.

Furthermore, scaling of non-ultrametric trees by the classic scale operator is known to be inefficient, and the TreeStretcher and TreeFlexer were introduced as a replacement. But, now we have the IntervalOperator in the TargetedBEAST package that performs even better (see preprint.) It makes sense for the default scale implementation on a tree to use the IntervalOperator approach to scaling. For ultrametric trees, nothing changes, but for the non-ultrametric trees the scaling will differ considerably. Dealing with HRs may need some attention.