mkdir stanfile edge cases

[hyunjimoon]

stanfile is not generated when

1. mdl file is in "/inventory/compare_vensim_stan.mdl" subfolder format

works:
vensim_model_path = "/Users/hyunjimoon/Dropbox/tolzul/BayesSD/code/pipeline/prescriptive/stanify/vensim_models/compare_vensim_stan.mdl"

not works:
vensim_model_path = "/Users/hyunjimoon/Dropbox/tolzul/BayesSD/code/pipeline/prescriptive/stanify/vensim_models/inventory/compare_vensim_stan.mdl"

2. stan_files folder should pre-exist under vignette folder before running notebook

e.g.