NixIO does not read channel_ids attribute of ChannelIndex

Example:
```python
>>> io = NixIO('test.h5')
>>> b = Block()
>>> b.channel_indexes.append(ChannelIndex(index=[1,2,3], channel_ids=[1,2,3]))
>>> io.write_block(b)
>>> b2 = io.read_block()

>>> b.channel_indexes[0].channel_ids
array([1, 2, 3])
>>> b2.channel_indexes[0].channel_ids
array([], dtype=int32)
```