Added fix for tritium decay

[lobis]

Fixes #67

For more information about the solution read https://geant4-forum.web.cern.ch/t/triton-decay-with-rdecay01-example/2616.

Tritium decay can now be used via the following option on the physics list:

<physicsList name="G4RadioactiveDecay">
    <option name="TritiumDecay" value="false"/>
</physicsList>

If the primary particle source is H3, then this option is enabled by default. It can still be explicitly disabled. It is disabled by default for other primary particle sources.

I could not make it work for old Geant4 versions (namely the 10.4.3 reference version). It can probably by implemented if anyone wants to do the fix.

[glorialuzon]

You should compare results: this activation versus the old one H3 source using an histogram. In the past the H3 decay of geant4 didn't produce any event. Using it as a source in the gas the full decay spectrum is expected.
Not sure if it should be also activated in case of neutron interactions (cosmogenic production)

[lobis]

You should compare results: this activation versus the old one H3 source using an histogram. In the past the H3 decay of geant4 didn't produce any event. Using it as a source in the gas the full decay spectrum is expected.
Not sure if it should be also activated in case of neutron interactions (cosmogenic production)

I have checked the spectrum and visually it looks identical to the one in the literature. I will share a screenshot in this PR later on.

I think the root histogram should be deprecated in favour of this approach as it seems more natural for users and it was only introduced as a workaround.