Introduce an Executable class (or similar)

[MichaelRoeder]

To improve the reusability of the ontology, it is important to keep the domain and range of properties open to other use cases. Properties like the parameters or the hobbit:imageName property are restricted to the union of system instances and benchmarks. However, they actually could be useful for every container that can be executed. Hence, a super class of system instances and benchmarks should be introduced (e.g., Executable) that represents a more generic class of resources that simply can be executed and may need some parameters.