Cleanup SubmitHGCalPGun.py parameters.

[riga]

The SubmitHGCalPGun.py script progressed quite much over time and so did the variety of parameters. Though their effect is documented (visible via --help), it would be helpful to be able to rely on naming consistency. Right now, there is upper-camel-case, lower-camel-case, and kebab-case (→).

Since all parameters are internally accessible by the name configured via dest entries, which should not be changed, this issue could be solved rather quickly. Most of the work would be needed to update the documentation accordingly.