Switches a completed fit's covariance to method. A previously
computed covariance is re-installed from the cache; otherwise it is
recomputed at the converged estimates: "r,s"/"r"/"s" and
"analytic" on a zero-iteration FOCEI model, and "sa" (SAEM
Louis FIM) / "imp" (importance-sampling Monte-Carlo) via the decoupled
recompute engine (the latter two require a mixed-effects fit). When
"sa"/"imp"/"analytic" cannot be computed the covariance
is left unchanged (it is never silently downgraded to "r,s").