Called by setRateFunction() to verify that a candidate rate function
returns an array (or vector/list) of the correct dimensions for the
requested rate.
.checkRateFunctionOutput(params, rate, fun)Invisibly NULL. Called for its side-effect of stopping with an
informative error if the output has the wrong shape.
A MizerParams object
Name of the rate being replaced, e.g. "Encounter".
Name of the candidate function to validate.