[Halla12_software] GEMC installation method updated

[Zhiwen Zhao]

Dear All

GEMC has updated its installation method to be more streamlined and have 
better version control
For SoLID using GEMC 1.8, it's now included in JLAB_VERSION 1.0, while 
GEMC 2.0 is currently under JLAB_VERSION devel
The instruction is updated here
https://hallaweb.jlab.org/wiki/index.php/Solid_install#GEMC

The new env script is checked in SVN at
https://jlabsvn.jlab.org/svnroot/solid/solid_gemc/script/set_solid
refer to
https://hallaweb.jlab.org/wiki/index.php/Solid_sim_geant4#Quick_start

Zhiwen