Fix bugs in ZarrIO to support roundtrip from HDF5 to Zarr

[oruebel]

Motivation

!!! Missing features:

• ZarrIO.write_dataset
  • H5ReferenceDatasets are currently not written when converting from HDF5 to Zarr
  • np.bytes_ datasets should be written as string type datasets rathern than JSON arrays

Changes made in this PR

• Replace build.written with def __buikder_written_to_zarr_io This is a bit hackky, but when we roundtrip from HDF5 (i.e., read from HDF5 and write to Zarr) the builders are already marked as written. This should ideally be resolved in the BuildManager
• Fix write of attributes:
  • Ensure numpy arrays with more than one element are correctly written as attributes
  • Ensure np.bytes_ (in addition to python bytes) are converted to utf-8 string arrays to make them JSON serializable
  • Ensure attributes with references to Builders are written correctly
• Handle np.dtypes when checking whether a dtype is a reference
• Hanlde np.dtypes when determining the data type
• For datasets np.dtypes only as objects (via JSON) if the np.dtype is not a np.number subtype
• When writing object datasets, convert np.bytes_ to utf8 strings to make sure they are JSON serializable
• In ZarrIO.listfill set the code (i.e., JSON) when serializing objects

How to test the behavior?

Using PyNWB from NeurodataWithoutBorders/pynwb#1018

from pynwb import NWBHDF5IO, NWBZarrIO
h5r = NWBHDF5IO('H19.28.012.11.05-2.nwb' , 'r')
f = h5r.read()
zw = NWBZarrIO('test_zarr_nwb', 'w', manager=h5r.manager)
zw.write(f)

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