Modernized the code for GoodSeedProducer

The reader arguments were being leaked if more than one Run is processed
in the job. Switching all pointer ownership to std::unique_ptr simplified
all memory handling in the class.
Removed to unused member variables: ps1Clus, ps2Clus.
Moved header into plugin directory.
Declare plugin directly in .cc file.
NOTE: The .h and .cc file can now be merged into 1 file, but I didn't
do this to avoid making code diffs more difficult.