[Feature]: Warn users if they request se for fixed quantities

[k-doering-NOAA]

Related to https://github.com/nmfs-stock-synthesis/ss-test-models/issues/3

@Rick-Methot-NOAA realized it would be useful to warn users if they request standard error vals for quantities that are fixed. This matters because it sometimes can be the only reason for an non positive definite hessian.

[Rick-Methot-NOAA]

I think the MG_active() vector could be used to check to see if M or growth is estimated ivector MG_active(0,7) // 0=all, 1=M, 2=growth 3=wtlen, 4=recr_dist&femfrac, 5=migration, 6=ageerror, 7=catchmult

there is no equivalent for the selparms, but there easily could be creation of a vector that was fleet-specific