Add bounds (or checks) to AdaptiveInterpolator

[aornugent]

Need to check that our spline doesn't go below zero, as identified in K93 with high values of k_I. Reprex to follow.

[aornugent]

This might be as simple as clamp-ing the result. Not sure what the overhead is. https://en.cppreference.com/w/cpp/algorithm/clamp