Fix minor issue when only one DARM digital filter bank is used

@joseph-betzwieser pointed out that when only one DARM digital filter bank is used (like at L1), there is a problem with indexing the filter banks. This update fixes the issue and does something similar to what we're already doing for the sensing paths. Digital filter bank names should be a list of strings, modules in use need to be a list of lists, and gains need to be a list.