Add "linear" (extrapolation) mode to map_coordinates

[timmens]

map_coordinates currently does not support linear extrapolation. This is in line with scipy's map_coordinates.

In JAX's implementation of scipy's map_coordinates, it is simple to add this functionality by, de facto, only adjusting the first line in the function _linear_indices_and_weights (jax.scipy.ndimage module); I've tested this already.

I can understand if JAX does not want to add features to scipy implementations not supported by scipy. Would you recommend opening a PR in scipy first in this case? (Although the change there might be non-trivial since JAX's version only implements a subset of the features provided by the scipy implementation)

[timmens]

Hello!

I've realized since first posting that it was suggested that the scipy.ndimage module is "out-of-scope" for JAX (see JEP 18137: Scope of JAX NumPy & SciPy Wrappers).

My follow-up question is: If I implement a custom map_coordinates with linear interpolation for my use case, do I have to include the JAX License in my package, or is a reference enough? Since I only need one of the many implemented cases, the code would look very different, but, of course, it would be motivated at least to some degree by the initial implementation.

Thanks a lot!

[jakevdp]

JAX is licensed under Apache 2.0 which has a few requirements for derivative work (wikipedia has a good summary: https://en.wikipedia.org/wiki/Apache_License#Licensing_conditions)