set the decoding version consistently at one place only and refer to that

The following discussion from !179 (closed) should be addressed:

• @apearce started a discussion: (+1 comment)

  Unrelated to this MR, but it's annoying how we bind a parameter of FTDecoding, but then have to make sure to pass the same value to gec 🙃 Ideally bind is used in such a way that you only need to set the value once.