Expand README example

[k-doering-NOAA]

Expand the example on the readme to illustrate using SSMSE for multiple scenarios.

• scenario 1: cod model as is as the OM, cod model as is as the EM
• scenario 2: cod model with higher M as the OM, cod model as is as the EM.

This will better illustrate to users how the run_SSMSE() function works and and show results when OMs and EMs are slightly different.

[k-doering-NOAA]

Used steepness instead of natural mortality, because M proved to be a very sensitive parameter and the EM had trouble converging when the OM's M was not the same as assumed in the EM.