Closes #151
How this PR improves usability for the developer (who updates fields with text editor) and user (who calls fields in Jupyter):

• The fields code is smaller as logic has been removed in some place
• Logic removed to check columns for square brackets, weight now passed as a named param
• "Check Columns" is a simple base method which checks within the method that the columns are as expected
  • A developer can "see" what is expected in the variables passed to the method
  • Previously the init_columns called by init_subclass which was called by inherited classes of the base was how this was done, and it was hard to understand what was happening
• Abstract base class setup maintained because prototype would required rewriting all the member variables and constructor for each "inherited". Member variables set up with @Property are clear
• Option to pass spin removed - this flexibility was overkill, so setting it in the class, away from the user, makes the implementation clearer.