Fstd2nc

* Some more compatibility for writing FST files with .to_fstd().
* Disable rlon adjustment for yin-yang grids that are combined with a
subgrid axis or stacked together. The rlon adjustment would be
different for each subgrid, so only one of the two grids was being
adjusted properly (bug).
* New --interp-degree argument for controlling interpolation degree
(experimental).
* Allow extra keyword arguments to fstd2nc.extra.get_crs, for tweaking
the crs object construction.
* Pixi package management (thanks to Sebastien Fortier).

* Fixed bug is to_fstpy that was causing strange interaction with
dask arrays.

* Fixed bug in longitude wrapping for 'L' grids.
* Fixed writing 'X' and 'O' grids back to FST.
* Some fixes for numpy 2.x compatiblity:
- Use np.prod instead np.product (latter is deprecated).
- Cast int32 addresses to int before doing operations.
numpy 1.x would auto upcast to int64 if there would be an
overflow, but numpy 2.x just lets the overflow happen.

* Fixed bug in reading files created by librmn 20.1-alpha.

* Fix bug when reading multiple files with masked records, when some
records are repeated across files.

* Fix bug when there's a mixture of 0mb and other pressure levels.
This was triggering some weird behaviour in the 'accum' mixin,
even if the fields weren't accumulated quantities.