transform_unary_ops now introduces aux variables/equations for all unary ops specified by UnaryOpNode::createAuxVarForUnaryOpNode()

In the absence of this option, if a var_model statement(s) is present, then aux vars/eqs are created for the same types of unary operators but only for equations specified in the var_model statement

In the absence of both this option and var_model statements, no unary op auxiliary variables are created

diffs continue to be substituted everywhere; for the moment auxiliary variables are created for diffs of expressions. A forthcoming change will allow auxiliary variables created for diffs of expressions to be linked with their lagged expressions as is currently the case for diffs of variables