Parameterize some settings variables in default_settings.yml

[mnlevy1981]

The default settings file (feisty/core/default_settings.yml) has blocks like

food_web:
  # [snip some intervening keys]
  # large demersal
  - predator: Ld
    prey: Mf
    preference: 0.375 #should be A*D = (school_avoid * generalist_reduct)

  - predator: Ld
    prey: Mp
    preference: 0.75 #should be D (generalist_reduct)

As the comments indicate, .0375 is really a generalist_reduct term times a school_avoid term, while the 0.75 is just the generalist_reduct term. We'd like to introduce a way to set those two parameters elsewhere in the file, and then have these food_web.preference terms change if the user changes the other parameters