Add fastGWA-GE module to fastGWA module

[wjzhong]

Dear Zhili and Jian,

We have proposed a fast and powerful linear mixed models (LMM)-based approach, fastGWA-GE, to test for genotype-environment interaction (GEI) effect and G+GxE joint effect. And we have developed the fastGWA-GE software on top of fastGWA module from gcta package. We have modified the following scripts. We mainly modified the FastFAM.cpp to add the fastGWA-GE module. We are wondering whether the fastGWA-GE module can be merged into the fastGWA module in the gcta package. Thank you very much for considering our pull request! Please let us know if you have any questions or suggestions.

README.md
include/Covar.h
src/Covar.cpp
include/FastFAM.h
src/FastFAM.cpp
include/StatLib.h
src/StatLib.cpp
src/main.cpp

Thanks,
Wujuan