Aim is to find somewhere between ease of use for a developer (removing confusing code in the module) and a "user/scientist" (adding confusing code like the square brackets when passing a weight)

The fields module is over-designed. Some things can be changed :

• init_columns (method called by base constructor) isn't really needed - there is an complicated mechanism to find the weights (based on finding square brackets in a string) with many lines of code
• init_subclass (called by child classes) is also a overly complicated way to pass spin to the class, we can default to 0 and update in a constructor for the rare cases of spin 2

I'm assuming not much need to create new fields on the fly by users, but there are some unit tests for this.

Testing :

• Regression tests (use existing) + test sample code works as expected - will require updates
• new tests to ensure Fields handle columns correctly and throw errors if the incorrect number is passed
• Understandable errors when columns / mapper are incorrect (let's see)