Standardize seqspec index and onlist output

[zhwshen]

1. seqspec index -m rna returns the read index string while seqspec index -m atac will return a "text text read_index" string. It'd be nice to have them return the same output format, either only return read index string or "text text read_index".
2. seqspec onlist returns a newly created onlist_joined.txt if the multiple onlist files in multiple barcode regions are present. It only echos back the URL/path/etc to the onlist file if no combination is involved. Is it possible to return a newly generated file regardless of whether there is a combinatorial step or not?

Thanks!

[zhwshen]

Actually for Item 2, it is resolved by adding -o <output file patch> even for no -combination