sv.RdCalculates steepness and virgin biomass given a and b for a Beverton and Holt SRR
# S4 method for FLPar,character
sv(x, model, spr0 = NA)FLPar with a and b
character with name of stock recruitment relationship, by default "bevholt"
spawner per recruit at F=0
any other arguments
FLPar with values for steepness (s) and virgin biomass (v)
if (FALSE) {
#bug
params=FLPar(a=37.8,b=8.93)
sv(params,"bevholt",.4)
}