RVL::LocalDataContainer - core abstraction, subtype of RVL::DataContainer, templated on Scalar type, with two essential pure virtual public methods to be implemented in subclasses:
In principle Scalar is very little restricted, and could be a complex type itself. In its intended use, Scalar is a number type with value semantics.
The eval methods inherited from DataContainer are implemented. This Element (Visitor design pattern) admits only two types of Visitor, namely RVL::LocalEvaluation and RVL::LocalConstEval (described in next bullet).