Operators cannot adapt DOP for NonParallelInputs

InputFormats that cannot be processed in parallel implement the NonParallelInput interface.
During optimization, the optimizer checks for this interface and sets the DOP of an operator to 1 if it is found. Other operators such as Mappers set their DOP during program construction to the DOP of their preceding task (if not specified otherwise). Since non-splittable data sources are only considered later by the optimizer, a Map operator will not have the same DOP as an preceding non-splittable data source.

The simple solution is to set the DOP of a non-splittable data source during program construction.