[NixIO] Support "empty" annotations

[achilleas-k]

Support empty lists in annotations (for NixIO).
See #476

[JuliaSprenger]

For there is a quick fix for this (#476) but I think we should keep this open to find a more general solution.

[JuliaSprenger]

Hi @achilleas-k
Any news on the support of empty annotations from the nix side?

[achilleas-k]

Took me a while to remember what the situation was here. As a reminder for myself and anyone else that might need this:

Currently the Neo NIXIO writes empty annotations with a string dtype because properties must always have a dtype:

python-neo/neo/io/nixio.py

Lines 1326 to 1331 in 4e90312

	if len(v) == 0:
	# NIX supports empty properties but dtype must be specified
	# Defaulting to String and using definition to signify empty
	# iterable as opposed to empty string
	values = nix.DataType.String
	definition = EMPTYANNOTATION

It uses the definition field of the property to mark it as an empty annotation and return an empty list as its value on read, because otherwise it would return an empty string:

python-neo/neo/io/nixio.py

Lines 1381 to 1383 in 4e90312

	if not len(values):
	if prop.definition == EMPTYANNOTATION:
	values = list()

Since (I think) v1.5.0b3, property values are always tuples, even when a single value is stored. It still requires the dtype to be specified, but when storing an empty value, the on disk representation is essentially an empty array. On read, this returns as an empty tuple.

So now we can indeed get rid of the workaround with the definition attribute. Storing an empty value, regardless of dtype, will be read back as an empty tuple, which we can then present to the Neo user as an empty list if necessary.

[JuliaSprenger]

Since there is a quick fix for this already existing, I am postponing the clean solution to the next release