Add mode= in most functions to capture the class out of bounds errors.

[rainwoodman]

Currently if we evaluate outside of the bounds an exception is raised. We can do better by handling this like scipy function, mode='raise'/'constant'/'nearest', and set the value accordingly without raising an exception. This becomes handy in interactive plotting.

[rainwoodman]

@energy-berkeley something like this

https://docs.scipy.org/doc/numpy-1.13.0/reference/generated/numpy.interp.html

but default to left='raise', right='raise' could work.