Not sure about model time step aggregation

@andydawson and @crollinson 

I just grabbed the biomass value every 100 years. But, should it be an average window? or should I use annual data like @andydawson did in her fcomp stability? 

https://github.com/PalEON-Project/EcosystemStability/blob/ab9f786037a0557d6fcefcee0e02cfe9efa27b1e/scripts/centennial-biomass-models.R#L129