Check for regular chunks before writing

[LucaMarconato]

If chunks are irregular (for instance we had problems due to this previous bug https://github.com/scverse/spatialdata/issues/821), an error is raised upon saving. We could catch this possible error early by moving the code to check for regular chunks (it's a private function from dask.array, see here https://github.com/scverse/spatialdata/pull/861) into RasterModel.validate().