Make the used sources explicit for tests.
Implement the Nugrammer class.

This class generalizes the histogramming so that different sources
of neutrinos can be separated.
Print more information about steps.

Improve the output of the stepping actions. Most importantly,
print out the list of all secondaries produces at every step.
This is printed only if `--track-verbose` is enabled.
Improve NeutrinoStackingAction debug output.
Implement Nugrammer in the main code.

This include a new class and user action - PostUserTracking. This
is used to store information about the parent particle (track).
Info is stored using UserTrackInformation.