Add getSolutionSet() and .getWorkset() for delta iterations

Before, the result of dataset.iterateDelta() would be the workset and
the solution set could be obtained via iteration.getSolutionSet().

This is counterintuitive for two reasons: 1. in BulkIteration the result of
the dataset.iterate() is the solution set. 2. you call .iterateDelta()
on what is to be the initial solution set and pass the workset as
an argument, so it makes sense that what you get as a result is also the
solution set placeholder.