getBlow: Blow parallel optimization function
In zanbi/DLMtool: Data-Limited Methods Toolkit

Find the current biomass at which it would take HZN mean generation times to reach Bfrac x SSBMSY biomass level given zero catches

getBlow(
  x,
  N,
  Asize,
  SSBMSY,
  SSBpR,
  MPA,
  SSB0,
  nareas,
  retA,
  MGThorizon,
  Find,
  Perr,
  M_ageArray,
  hs,
  Mat_age,
  Wt_age,
  R0a,
  V,
  nyears,
  maxage,
  mov,
  Spat_targ,
  SRrel,
  aR,
  bR,
  Bfrac = 0.5,
  maxF,
  ploty = F,
  plusgroup = 0
)

`x`	position in a vector
`N`	array of numbers at age by sim and area (dims nsim, maxage, nyears, nareas)
`Asize`	matrix of area size (nsim by nareas)
`SSBMSY`	vector nsim long of spawning biomass at MSY
`SSBpR`	matrix nsim by nareas with SSB per recruit
`MPA`	matrix nyears + proyears by nareas of spatial closures
`SSB0`	vector of SSB0 length nsim
`nareas`	numeric. number of areas
`retA`	array of retention nsim x maxage x nyears
`MGThorizon`	vector nsim long of MGT x HZN
`Find`	matrix of fishing mortality rate nsim x nyears
`Perr`	matrix of recruitment devitions nsim x nyears + maxage -1
`M_ageArray`	array of natural mortality rate nsim x maxage x nyears + proyears
`hs`	vector nsim long of steepness values
`Mat_age`	array nsim x nages x nyears+proyears of maturity at age
`Wt_age`	matrix nsim x nages of weight at age
`R0a`	matrix nsim by nareas of unfished recruitment
`V`	array of vulnerability nsim x maxage x nyears
`nyears`	integer: number of historical years
`maxage`	integer: maximum age
`mov`	array of movement nsim x 2 x 2
`Spat_targ`	vector of spatial targetting parameters
`SRrel`	integer representing recruitmetn dynamics type 1: Bev Holt 2: Ricker
`aR`	vector of recruitment parameters
`bR`	vector of recruitment parameters
`Bfrac`	fraction of SSBMSY that is the target
`maxF`	maximum apical fishing mortality
`ploty`	logical: should a plot be produced
`plusgroup`	Integer. Default = 0 = no plus-group. Use 1 to include a plus-group