Create output dir.
See also
Other polyfun:
POLYFUN(),
POLYFUN_compute_priors(),
POLYFUN_download_ref_files(),
POLYFUN_find_folder(),
POLYFUN_finemapper(),
POLYFUN_gather_annotations(),
POLYFUN_gather_ldscores(),
POLYFUN_help(),
POLYFUN_import_priors(),
POLYFUN_munge_summ_stats(),
POLYFUN_prepare_snp_input(),
POLYFUN_run_ldsc()