Support for other resampling types.

[SpacemanPaul]

Current behaviour is to always use "Nearest Neighbour" resampling. But low level internal APIs already pass around resampling type as a free parameter, so supporting other resampling types would be a relatively straightforward enhancement.

I believe WCS (at least version 2) supports a resampling mode as a user-requested parameter. For protocols that don't support resampling mode as a user-requested parameter (e.g. WMS?) a default could be supplied in config per layer or per style.

Nobody has asked for this, and I'm not sure if there's any real use case for it - but if anybody finds one, it would be a pretty low-effort enhancement.