Add the possibility to specify negative PIDs.
Add decay channels for metastable hadrons and mesons.

These channels should be used only with Geant creator, but for the sake
of completeness they can be generated with Pythia also.
Add support for single particle insertion (with Geant4).

Also reorganize some stuff related with this change, such as:
- clean up the `get_primary_generator_action` function
- rename the Geant4 creator class
- update to the `-c` argument