Explicitly define groups in the configuration file

[Victor-Savu]

While the tolerance parameter offers a very succinct way to uniquely define the groups of lines to be fitted jointly, it requires some validation on the part of the user to make sure there are no surprising groupings.

Here are two things we could consider to avoid any confusion:

1. A sub-command (gleam list groups) to list the groups (for each override scenario) so the user can easily inspect them.
2. Explicitly define the groups in the config file, using line names:

   groups:
    - 
      - Halpha
      - NII1
      - NII2
    -
      - SII1
      - SII2

   All other lines would be fit individually.