please document _rlnHelicalTubeID as _ibIceGroup

Please update the README to reflect that the header tag with the ice breaker score when using ib_job.py is
in stored in _rlnHelicalTubeID

the only reference to this is in comment in the file blow

https://github.com/DiamondLightSource/python-icebreaker/blob/master/README.md

https://github.com/DiamondLightSource/python-icebreaker/blob/0fa6e1af5ee3f5a6d00fe5cd37da60fe65127953/src/icebreaker/star_appender.py#L36

Please also update the loading of settings

`Copy one of the templates to your Relion project directory as .gui_externaljob.py`

this needs to be called .gui_externaljob.star to be loaded correctly into the relion gui