Support for 1D LUT extrapolation

We've come across situations where it would be nice to support some form of extrapolation of 1D LUTs, or more generally to control the behavior outside the domain/range of the LUT files.

The most obvious approach would be to add a keyword to control this option in the config, care should probably be taken to not paint the system into a corner by considering what options may be needed if say cubic interpolation was added in the future (preserving 1st derivative, 2nd derivative options etc).

Default behavior would most likely be to clamp as is done currently.

Kevin