The fitted models must be of a class for which there is a logLik
method (e.g., `secr' or `lm'). Check with methods("logLik").
The models must be nested (no check is performed - this is up to the
user), but either model1 or model2 may be the more general model.
The models must also be compatible by the criteria of AICcompatible.
The test statistic is twice the difference of the maximized
likelihoods. It is compared to a chi-square distribution with df equal
to the number of extra parameters in the more complex model.