Use the built-in intergalactic material for vacuum.
Use a more realistic world geometry.

This include:
* Modeling the world as a shell from earth's surface up to Fermi's altitude.
* Using less dense atmosphere for the first 100 km and vacuum for the rest.
* Removing the pressure factor option.
* Update the way the material for the atmosphere is created.
Remove the legacy fMaterial member.
Remove an out-of-date comment.
Output more info to events.txt per event.