Warning for incompatibility in common_lib utility function

[robclewley]

common_lib was updated a couple of times during the semesters I taught this to quickly patch some emerging problems the students had. As a result, the extant version may not quite work with some earlier or later chapter codes. This will be due to changes in thresh_Naka_Rushton_fndef and the use of the "with_if" option. Chapter codes may need to be updated to use this option explicitly in order to either protect against spurious effects of negative inputs (with_if=True) or to allow symbolic Jacobians to be automatically computed (with_if=False), provided the user ensures no negative inputs.