Implement reading event source data from ROOT file

[lobis]

Currently the only way to read external data to generate a custom source is through a .dat file.

I would be nice to extend TRestGeant4Metadata::ReadEventDataFile to also read root files containing a tree which defines the source and generates a TRestGeant4ParticleCollection just like TRestGeant4Metadata::ReadNewDecay0File.

[nkx111]

But we need to define a format of root file as generator