In addSpecies() only issue warning if feeding level drops below critical level

[gustavdelius]

Currently, if one of the newly-added species has predation parameters that lead to a feeding level that drops below the critical feeding level, the function aborts with an error message: "Can not compute steady state due to zero growth rates for ...". This should be changed so that an error is issued only if the growth rate becomes zero already before the species reaches maturity size. If the species does make it to maturity before growth stops then only a warning should be issued. That will make it easier for the user to make adjustments to the new species interactively with tuneParams().