Add optional YAC remapping backend with NNN/conservative support and CI coverage

[rajeeja]

Closes #1441, Close #897

• Added a YAC-backed remapping backend so UXarray can use YAC’s interpolation machinery when requested, while remaining entirely unchanged for existing UXarray workflows. The UXarray remap API now accepts backend="yac" plus yac_method and yac_options. If YAC isn’t available, throw error.
• Implemented YAC NNN (nearest‑neighbor) remap using YAC’s InterpolationStack.add_nnn, mapping UXarray grid locations (nodes/edges/faces) to YAC cloud grids (point-based) and remapping via YAC’s coupling API.
• Implemented YAC conservative remap for face‑centered data only, using YAC unstructured grids with face‑node connectivity. This preserves the correct conservative semantics for cell‑centered fields.
• Built a YAC-to-UXarray conversion path that:
  • Converts node coordinates to radians for YAC,
  • Constructs YAC UnstructuredGrid from UXarray face_node_connectivity,
  • Uses YAC points at the appropriate locations (CELL for conservative, CORNER for cloud).
• Added a backend parity test to compare UXarray NN and YAC NNN on a tiny deterministic 3‑node grid. This avoids tie‑breaking ambiguity and provides a direct sanity check that YAC’s nearest‑neighbor agrees with UXarray’s own implementation.

[hongyuchen1030]

Can you be more specific here about what's "remap"? Is it conservative or non-conservative? You can reflect it either in function name or comment

[hongyuchen1030]

Looks good! Just add one comment about bit more comment description. Thanks

[k202077]

In your application the setup of YAC should get much easier (e.g. no setting of calendar or start/end time required) by using the (unfortunately basically undocumented) Python bindings for the yac_core library.

Here is some basic documentation:
https://dkrz-sw.gitlab-pages.dkrz.de/yac/d8/d22/core_8pyx.html

This is an example using this interface:
https://dkrz-sw.gitlab-pages.dkrz.de/yac/d8/d58/test__core_8py_source.html

You can configure YAC to only build the yac_core library (which does not depend on libfyaml). In addition libnetcdf is also optional.