Implementation error parameters when stddev=0

Old versions of SS3 included implementation error parameters in the forecast for almost all scenarios. In the new versions of SS, implementation parameters are not included unless stddev is >0 in the forecast file. This is causing some old SS models to crash with no helpful debugging output regarding why. We can either catch this in the code and remove the parameters if needed or at least add a warning in the documentation/examples to add a value >0.