Replace an NA or invalid value in a slot of a Panel object with the first valid choice.
This is usually called in .refineParameters.
.replaceMissingWithFirst(x, field, choices)An instance of a Panel class.
String containing the name of the relevant slot.
Character vector of permissible values for this slot.
x where the slot named field is replaced with choices[1] if its value was previously NA or did not exist in choices.