WIP: Add soundfiler outlet squashed

supercedes !449 (closed) where I erroneously branched from another feature branch.

• need some sane coverage tests. Should be pretty easy to enumerate through all the errors for read and write
• need some tests for the right outlet
• need to do a few manual sanity tests reading and writing soundfilers with both soundfiler and writesf~
• need to squash again before merging