Fixed `read_seq` for reading v1.3.1 sequence files

[FrankZijlstra]

• Removed some references the to lengths field in event_lib. I removed this field in previous PRs because it was completely unused, but missed these uses.
• Fixed the handling of block durations and delays for 1.3.1 sequence files in read_seq.

Should fix #161

[FrankZijlstra]

In general, the types in event_lib are not all correct and the employed update() method doesn't require old data, which you just consequently set to None here , but this is something we can discuss during the code refactoring process and not here I guess.

When I removed the use of old_data before, I did consider just removing it altogether, but figured that it would be a good idea to keep compatibility (in the unlikely case someone was ever adventurous enough to mess with the event libraries...). But I think it's pretty safe to remove altogether.