Support reading `nex5 ` files with NeuroExplorer

[h-mayorquin]

The current NeuroExplorer can only read the old format and with some limitations. See comments here:

#1773 (comment)

We first require support in Neo for the new5 (I am not aware if they added it yet) and then propagate these changes here.

Follow up on this:
#1093

And also linking to upstream needs on:
catalystneuro/neuroconv#216

[zm711]

Do you think someone could share a nex5 file with us. Maybe one of the labs you work with. The issue we have is always test files. But if we get a test file then we can work on it at the Neo level.

[h-mayorquin]

What about the ones in gin?
https://gin.g-node.org/NeuralEnsemble/ephy_testing_data/src/master/neuroexplorer