Hints for writing POWHEG-BOX-V2 dkfiles

The main writing format should be similar to Madgraph, you could find an example, while there are some points worth to be mensioned:

• The parton map provides the index to the number of final state particles (NPartons) of all POWHEG processes, which would be used in pythia8 for showering

• For some processes, e.g. ttb_dec, ttb_NLO_dec, the default nPartons is set to 10 while it should be 3, which is wrong, please check the NPartons before use

• We should be able to set the NPartons in POWHEG configurations.