Prevent FutureWarnings due to new xarray defaults

[veenstrajelmer]

New defaults for xarrays concat/merge/combine were set in pydata/xarray#10062. This is from xarray 2025.08.0 onwards.

These might result in futurewarnings like in Deltares/dfm_tools#1251, there are some ways to work around these:

1. setting the option in __init__.py breaks all older xarray versions, so that is not desireable
2. setting compat explicitly in all xarray merge/concat calls (that give a warning)
3. temporarily suppress the warning via pyproject.toml, but that still gives a warning for users.