Many experiments require pre-existing data sets for their execution.
This class represents such an association for its parent SimDM/Experiment. 
It is assumed to contain 1 or more objects of a specified SimDM/InputDataObjectType defined on the SimDM/Protocol used by the container SimDM/Experiment.
In those cases where the actual input data set is represented by a SimDM/OutputDataset, for example in a SimDB, we can represent the input data by a corresponding reference.  Whereas that would be optimal, it may not always be practical. In cases where this is not so, this reference is replaced with an accessURL attribute on this class. This would allow a user to find out whether a product exists in the database with that same url, make the link indirect but arguably more correct. 
 