[EventGeneratorAlphaDecay] Force the GlobalTime to random following the decay law associated to the activity of the sample

[EventGeneratorIsotropic] add the Flat source profile option in EventGeneratorIsotropic. By default, Gauss is chosen

histogram

        - NPNucleus will be renamed subsequently to NPParticle

  - ROOT branches with real energy/time added to NPLib
  - N.B. electron source added to EventGeneratorIsotropic

	- Someone commented out the generqtor for some reasons

        - The step could take a negative step length, leading to abort
        of event