[feature] GrasSim: Add cumulated sum of exported biomass to management attibutes

Summary

Add cumulated sum of exported biomass to management attibutes for total BM production visualization. reset to zero each year.

management.py :

Line 104-105 :

if self.day.dayofyear == 1:
            self.cum_exported_BM = np.zeros(self.grid)

Line 360 :

self.cum_exported_BM += self.exported_BM