Consolidate writer functions

[jo-mueller]

This is written based upon th state of the code in #516

In this PR, ome-zarr-py has two entrypoints to writing data, notably:

• write_image: Calls the private method _write_dask_image, which

  • uses scale._build_pyramid to generate a list of pyramidal delayed dask images
  • writes these to disc
  • writes the metadatato disk with write_multiscales_metadata

• write_multiscales_image: Requires a pre-computed or delayed image pyramid and

  • writes the levels to disk
  • writes the metadata to disk with write_multiscales_metadata

There is some obvious duplicity between both entrypoints. I suggest to:

• Move _build_pyramid up from _write_dask_image into the write_image function. That way, write_dask_image would be demoted to a function to simpl write pyramidal dask data and associated metadata
• That would make the contents of _write_dask_image and write_multiscales practically identically, so _write_dask_image could also be used inside write_multiscales.

This would greatly improve maintainability and condense the code down to a single location for writing were metadata for different ome-zarr versions would be handled.