code: public <T extends Number> Variable addInput(String name, T from, T to, T step) {
  		return addInput(new Variable(name, from, to, step));
  	}
docs: Add an input variable to this program.
  <p>
  @param <T>  the type
  @param name the variable name
  @param from start of interval
  @param to   end of interval
  @param step the step
  @return the variable