ABF: raise a clear error on files with an invalid signature

[h-mayorquin]

A file whose header does not start with a valid ABF (Axon Binary Format) signature (b"ABF " or b"ABF2") failed with a cryptic TypeError: 'NoneType' object is not subscriptable deep inside parse_header, because parse_axon_soup returned None for an unrecognized signature and the caller then indexed into it. This now raises a clear NeoReadWriteError naming the file and the unexpected signature, so a caller can tell a corrupt or unsupported file from an actual bug.

I put the error in parse_axon_soup at the point the invalid signature is detected rather than null-checking in the caller, because that is where the signature bytes are available for the message and it makes "an unparseable file always raises" a property of the parser itself. The fixture unparseable_header.abf (a zeroed signature) covers it.