add bfgs interface with a little doc
update stan to latest commit
add refresh to the doc of optimizing
Merge branch 'develop' into feature/add_bfgs
rstan revise vignette
bfgs prints out info abot last iteration
working on the vignette
use int for loop index to rm 2 warnings from compiler
Merge branch 'feature/add_bfgs' into develop
Merge branch 'feature/reduce_warning' into develop
Merge branch 'feature/vignette' into develop