Add GiGaMTPhoenix

Needs #14. This MR adds a new package GiGaMTPhoenix that can be used to export simulation information to JSON. It can be then opened in Phoenix, the experiment-independent visualization framework. DumpG4Trajectories is the first example. It dumps the G4TrajectoryPoints as position coordinates that will be later extrapolated using Runge-Kutta method (by Phoenix) when passed as Phoenix::Tracks. Additional optimization options and python configuration is needed.

@fbilandz we might want to experiment with this example a bit to see what other data containers can be visualized, e.g. hits in LHCb event model (if it makes sense) etc., and also what else needs to be done in Phoenix and Gaussino

cc: @bcouturi, @sponce, @gcorti

Cube example

I tried to read the geometry and trajectories using a very simple example from https://gaussino.docs.cern.ch/master/examples/external_detector.html#external-cube. It consists of one photon hitting 1m x 1m x 1m cube made out of lead.