Parse molecules from SDF/MOL text block.
Examples
if (FALSE) { # is_installed_rdkit()
str_single <- readr::read_file(tidychem_example("single.mol"))
str_multiple <- readr::read_file(tidychem_example("5ht3ligs.sdf"))
parse_sdf(str_single)
parse_sdf(str_multiple)
}