The user specifies the probability of staying in the same area and spatial
heterogeneity (both in the unfished state). This function returns the
squared difference between these values and those produced by the three
logit movement model.

`movfit_Rcpp(par, prb, frac)`

## Arguments

- par
Three parameters in the logit space that control the four
probabilities of moving between 2 areas

- prb
User specified probability that individuals in area 1 remain in
that area (unfished conditions)

- frac
User specified fraction of individuals found in area 1 (unfished
conditions)

## Details

This is paired with getmov to find the correct movement model.

## Author

T. Carruthers with an amateur attempt at converting to Rcpp by A. Hordyk (but it works!)