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.

changed title from set the decoding version consistently in one function to set the decoding version consistently at one place only and refer to that

suggestion:

@configurable
def ft_decoding_version(value):
    return value

# use ft_decoding_version() everywhere where needed

with ft_decoding_version.bind(4):  #
    # ...

mentioned in merge request !233 (merged)

closed via commit 156e3446

mentioned in commit 156e3446

mentioned in commit LHCb@6c9511b2