Clean up use of @A and @sc slots

[gustavdelius]

There are some weird things in the code that made sense at the time they were introduced but need to be either documented or removed:

• The @A slot was originally intended to hold the observed biomasses of the species. This is now in the species_params data frame in the biomass_observed column. Now the @A slot is still used to determine whether a species is a background species. This too should be moved to the species_params data frame.
• The @sc slot plays an undocumented role in adjustBackgroundSpecies(), by holding the desired total fish spectrum. I am sure this can be handled more transparently.