[Halla12_software] another point about geometry API.

[Zhiwen Zhao]

Dear All

Another point about geometry API.
Our simulation needs to support both individual detector R&D and the whole SoLID simulation.
And it needs fast simulation besides full simulation in the same framework.
So having a geometry API, no matter what format it is, instead of hard code many things into source 
code is essential to make it effortless to unify different types of simulation in one single framework.
Other collaborations have done something similar and this should one thing we try to understand how 
they did it.

Zhiwen