Add protection against missing PDF information

Check a pointer to PDF information for validity before accessing details
from it. It a dataset is produced by a particle gun, the PDF information
is not available, and without the protection code fails with a segfault.
Add a plugin to store generator-level jets
Change counters in GenJetsInfo to more compact types

It saves 6 bytes of storage per jet and 3 bytes per event on top of it.
Add fillDescriptions method for the GenJetsInfo plugin
Update documentation for GenJetsInfo