Generate NARR url from a vector of dates

Figures out file names for the given dates, based on NARR's convoluted and inconsistent naming scheme.

generate_narr_url(dates, flx)

Arguments

dates	Vector of dates for which to generate URL
flx	(Logical) If `TRUE`, format for `flx` variables. Otherwise, format for `sfc` variables. See [narr_flx_vars].