test
merge from Kun
ad atoms example
test for Dyson
merge from Kun
find the bug in calculate_Sigma_first_order. consistent with Fortran code now
test for self-consistent loop
refactor the calculator
Merge branch 'dev'
Merge branch 'master' of github.com:kunyuan/Feynman_Simulator
refactor the spin index
Merge branch 'master' of github.com:kunyuan/Feynman_Simulator
cleanup
improving