utils.format_for_regrid requires uniform chunk sizes

[BSchilperoort]

https://github.com/xarray-contrib/xarray-regrid/blob/1425e1353b4405b4ea734ef37f78cf4538a72949/src/xarray_regrid/utils.py#L252C5-L252C37

obj.chunksizes is used here to coerce a coord back to a single chunk if that's what was passed. However, the obj.chunksizes will fail if the chunk sizes are not uniform (which can happen from loading a multi file netCDF).

A fallback here would be better than an error, which is the current behavior.

@SarahAlidoost