Rethink if-else-classifier?

[jmmcd]

With #145 leading to a new design for categorical features, the if-else-classifier https://github.com/PonyGE/PonyGE2/blob/master/grammars/supervised_learning/if_else_classifier.bnf would become an anomaly, possibly confusing to new users. It uses integers as categorical features, and special variables n_is and n_os (number of input symbols and output symbols), following the terminology in Moraglio et al PPSN 2012.