Timescale should also support fs

The `timescale` parameter is currently checked with a regex like so:

    if timescale is None or re.fullmatch("\\d+[npu]?s/\\d+[npu]?s", timescale):

Unfortunately, this regex is wrong as `fs` (femtoseconds) is also a valid timescale.  Seems `ms` has the same issue.  It's probably worth checking against relevant documentation to make sure this regex doesn't need to be updated again.