Axon: improve channel name handling

[h-mayorquin]

This improves how AxonRawIO builds ADC channel names, which previously used .replace(b" ", b"") and had three shortcomings. Names now come back as str rather than bytes; only the fixed-width padding is stripped so interior spaces are preserved (a name like "IN 1" was being mangled to "IN1"); a channel whose stored name is blank now falls back to a positional ch{id} instead of an empty "" that leaves it unaddressable; and decoding uses errors="replace" so an odd byte in a name can never crash the read.

One intended behavior change: files whose channel names contain interior spaces now report the real name, for example File_axon_6 and File_axon_7 return "IN 1" instead of "IN1". The space is part of the name Clampex recorded, so this is the correct value; any downstream code that matched the old space-stripped form should use the real name. The blank-name case is covered by the fixture abf1_episodic_empty_channel_name.abf.