mcmc_negfc_sampling: improve folder creation
mcmc_negfc_sampling: improve folder creation

the try/except prevents race conditions which could occur when chaining the two system calls `os.path.exists()` and `os.makedirs()`.
`datetime.datetime.strftime` is used for the output filename creation.

henry-ngo/VIP,vortex-exoplanet/VIP,carlgogo/vip_exoplanets