Revise option lazy_inputs_interval

[schillic]

The option :lazy_inputs_interval accepts integers. Internally, we only work with functions, and we convert such integers to functions. Up to now, we either use lazy inputs for the whole time, or we never use them. Still, the more general functionality makes sense with non-template directions (e.g., for epsilon-close approximation).

I suggest to split this option into two: :lazy_inputs of type Bool and default value false, and :lazy_input_predicate of type Function and default value k -> false (always use lazy inputs). This saves the internal conversion and clarifies the user interface. Usually you only want the Bool option.

Alternatively, we could only offer the option :lazy_input_predicate but also provide default predicates that the user can choose from.