Calculates the sum squared difference between log(N) in current and previous
state. This function can be used in `projectToSteady()`

to decide when
sufficient convergence to steady state has been achieved.

distanceSSLogN(params, current, previous)

## Arguments

params |
MizerParams |

current |
A named list with entries `n` , `n_pp` and `n_other`
describing the current state |

previous |
A named list with entries `n` , `n_pp` and `n_other`
describing the previous state |

## Value

The sum of squares of the difference in the logs of the (nonzero)
fish abundances n:
`sum((log(current$n) - log(previous$n))^2)`

## See also