Deprecated and likely not to come back, only really possible with one fleet, one critter, and no space, so in other words not what marlin
is intended for
Note that this is mostly an internal function to assign_ref_points
Usage
find_msy(effort_mult, fauna, fleets, opt = TRUE, target_critter)
find_msy(effort_mult, fauna, fleets, opt = TRUE, target_critter)