fish2weight: Calculate weight from fish age
In remi-daigle/BESTMPA:

Wrapper function for age2length and length2weight to estimate weight in each cell from fish.

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fish2weight(fish, ages, l_to_w_int = 1.1e-05, l_to_w_power = 2.91,
  Linf_mean = 112.03, Linf_SD = 10.46/1.96, k_mean = 0.13,
  k_SD = 0.021/196, t0 = 0.18)

`fish`	a matrix of fish numbers by site (rows) and by age (columns). Can easily be generated by `initpop`
`ages`	The ages of the fish of the fish to be converted to weight (year)
`l_to_w_int`	The intercept in the Length-weight relationship
`l_to_w_power`	The power in the Length-weight relationship
`Linf_mean`	Von Bertalanffy growth model parameter - mean asymptotic length (cm)
`Linf_SD`	Von Bertalanffy growth model parameter - standard deviation asymptotic length (cm)
`k_mean`	Von Bertalanffy growth model parameters - mean growth coefficient (1/year)
`k_SD`	Von Bertalanffy growth model parameters - standard deviation growth coefficient (1/year)
`t0`	Von Bertalanffy growth model parameters - x intercept (year)

# create a fish matrix
fish <- initpop(initial_abun=250*10^6,cells=length(BESTMPA_domain),maxage=50,rate=0.7)
# calculate weight
fish2weight(fish,ages=0:20,l_to_w_int=0.000011,l_to_w_power=2.91,Linf_mean=112.03,Linf_SD=10.46/1.96,k_mean=0.13,k_SD=0.021/196,t0=0.18)