Test for very long decay time

[dsarrut]

In G4 11.2, there is a change in the decay time threshold https://geant4-userdoc.web.cern.ch/UsersGuides/ForApplicationDeveloper/html/TrackingAndPhysics/physicsProcess.html?highlight=thresholdforverylongdecaytime#note-on-the-time-threshold-for-radioactive-decay-of-ions

For Cs137 (30 year half life), it leads to no decay. User must change it with: /process/had/rdm/thresholdForVeryLongDecayTime 1.0e+60 year

Add a test and add in doc

[BishopWolf]

I shall comment, I have had issues with times longer than 1e3 years, specially for beta emitters that contain a semi-stable alpha emitter as a child. The most notable case is Ac-225 which decays into Bi-209 through an alpha chain. The problem is that Bi-209 decays in 1.9e19 years to Tl-205 with an alpha of 3.14MeV which basically doubles the energy of all the other emissions. If we consider local energy deposition, this leads to an error of 200% in energy deposited while integrating in time. So be careful with this threshold.