[x] Instead of fixed bounds, we should count down from the OA. Wherever extra_mortality() is depended on.
[x] use OAG in fitting if the user includes it in extrapFit, i.e. allow override if it's part of vector.
[x] better defaults depending on the available age ranges for fitting. If it's lower ages, we prefer gompertz, if we get into higher ages then kannisto is ok.
extra_mortality()
is depended on.extrapFit
, i.e. allow override if it's part of vector.