Recorders with non voltage (non standard) channels and slicing channels

In #761 we move the warnings of non standard units to get_traces. We achieved this by introducing an extra attribute:

https://github.com/SpikeInterface/spikeinterface/blob/486bc67d364745437ebf1ed90bfdef7fae255c1c/spikeinterface/core/baserecording.py#L109-L116

However, as far as I know this attribute (self.has_non_standards_units) is not copied when we slice the channels with recorder.slice_channel which indicates problems. I think that what would happen right now is that the attribute NeoRawIOClass is not copied either so the sliced recorder with non voltage units will never raise the warning.

Maybe we should handle this with annotations as you already have the machinery in those methods to copy that?

Context: #758, #761 and https://github.com/NeuralEnsemble/python-neo/issues/1133

[h-mayorquin]

All right, so we should:

1. Do this in NeoBaseRecordingExtractor as @alejoe91 points out, and
2. Do this with attributes so the copying machinery of slicing works.

Please say if you agree with both and I will take care of it,

[h-mayorquin]

@alejoe91 we should probably move this warning to get_traces to the neobase recording withing the library.