Parses an atom_mask, which is string consisting of the @ symbol followed by a comma separated list of indices or ranges into a table containing all the indices it represents. E.g. @1,37-39,54,99-102 is expanded to a table with 1, 37, 38, 39, 54, 99, 100, 101, 102 as its first integer column. There must be no spaces in the mask.