Internal function to calculate MSY Reference Points

```
MSYCalcs(
logF,
M_at_Age,
Wt_at_Age,
Mat_at_Age,
Fec_at_Age,
V_at_Age,
maxage,
R0x = 1,
SRrelx = 3L,
hx = 1,
SSBpR = 0,
opt = 1L,
plusgroup = 1L
)
```

- logF
log fishing mortality

- M_at_Age
Vector of M-at-age

- Wt_at_Age
Vector of weight-at-age

- Mat_at_Age
Vector of maturity-at-age

- Fec_at_Age
Vector of mature weight-at-age

- V_at_Age
Vector of selectivity-at-age

- maxage
Maximum age

- R0x
R0 for this simulation. Set = 1 if SRrelx = 3 for per-recruit calculations

- SRrelx
SRR type for this simulation. Use 3 for per-recruit calculations, i.e. constant recruitment.

- hx
numeric. Steepness value for this simulation. Not used if SRrelx = 3.

- SSBpR
numeric. Unfished spawners per recruit for this simulation. Not used if SRrelx = 3.

- opt
Option. 1 = return -Yield, 2= return all MSY calcs

- plusgroup
Integer. Default = 0 = no plus-group. Use 1 to include a plus-group

See `opt`