ICB inconsistent naming in configs - needs to be rationalised

[drjzhn]

There is 'nel', 'nel_icb', NEL which are used as flags in different parts of code, including for directory naming.

Two competing sources for icb_name:

• The YAML config files set it to "NEL"
• The config_utils.py sets to "nel_icb"

No need to be this complex, need to rationalise to simple naming that is consistent everywhere.