Add filter for missing CHI

[Moohan]

Could be done like the other filters i.e. filter(!is.na(anon_chi)) but since all the missing CHIs appear at the start of the file it might be better to have an index of row numbers (which would need to be kept up to date - this could be checked with tests). Then the index could be used to do read_fst(from = <first_row_with_non_missing_chi>