NixIO does not support writing of numpy array_annotations of dtype datetime

[JuliaSprenger]

Describe the bug An object with an array_annotation of dtype=datetime.datetime can not be saved as this raises an error in nixio. _nixio_write_property needs to be adjusted to convert datetime objects in array annotations before saving these.

To Reproduce

import datetime
import numpy as np
import neo
import quantities as pq

block = neo.Block()
block.segments.append(neo.Segment())
event = neo.Event([1,2,3]*pq.s)
event.array_annotate(dates=np.array([datetime.datetime.now()]*3))
block.segments[0].events.append(event)

with neo.NixIO('testfile.nix','ow') as io:
    io.write_block(block)

Expected behaviour The file should be saved without errors.

Environment:

• OS: Linux
• Python version 3.8
• Neo version: current master
• NumPy version 1.20.3
• NixIO version 1.5.3

Additional context We should check that all allowed annotation types are correctly saved when used as numpy dtype. The full list of types can be found here