Fix-ate plexon signal streams

[h-mayorquin]

This PR solves two things: 1) Should solve https://github.com/NeuralEnsemble/python-neo/issues/1523 by using consistent stream_ids defined by plexon channel prefixes that are stable (see here 2) It adds human readable and meaningful stream names. Right now they are called "Signal + {stream_id}" and after this PR, they will be:

            channel_prefix_to_stream_name = {
                "WB": "WB-Wideband",
                "FP": "FPl-Low Pass Filtered ",
                "SP": "SPKC-High Pass Filtered",
                "AI": "AI-Auxiliary Input",
            }

Not that this moves forward the poject in https://github.com/SpikeInterface/spikeinterface/issues/3197 that aims to have "logical streams" and relegate "buffer streams" to an internal implementation detail of the raw io API.

[h-mayorquin]

OK, the test file that we have is quite limited. I would need this: https://gin.g-node.org/NeuralEnsemble/ephy_testing_data/pulls/137

[zm711]

OK, the test file that we have is quite limited. I would need this: https://gin.g-node.org/NeuralEnsemble/ephy_testing_data/pulls/137

Merged @h-mayorquin

I'll re-run tests so you can confirm your changes work with the new demo file.

[zm711]

The rtd failure should also be looked at. (Since plexon is used for the doc build!)

[h-mayorquin]

Fixed the test and added a new one.