nlmixr2saveFitIdentity() identifies which fit this is: the model, the
estimation method, the estimates and the data. It is the same for a fit,
its saveFit()/loadFit() reload, and the fit after addCwres().
Details
nlmixr2saveFitKey() additionally changes whenever something a downstream
result could depend on changes: added table columns (addCwres(),
addNpde(), addTable()), objective function rows (setOfv()), the
covariance (preconditionFit()) or the table values. := hashes fits on
its right-hand side with this key, so a result built on a fit reloaded in a
new session is still a cache hit.
Neither key forces or writes anything in the fit.