Read JetDS configuration from chainDict
Avoid parsing the chain name in favour of querying the chainDict. This was never read from the jet chainParts, so remove it and the defunct 'TLA' expression. Reading from the full dict rather than chainParts is preferable in any case, as it is then defined only once per chain, regardless of how many jet legs are present.
As gscThreshold no longer exists, I also took advantage of this opportunity to remove the lines that parse it.