Stop misusing TableMeta.dataset reference and make it read-only

[Marigold]

TableMeta.dataset should be a reference to DatasetMeta, but it's currently a cached copy that might be overwritten. This has caused confusion and we had to implement weird hacks to make it behave.

It's misused in several places if you search for metadata.dataset = (probably doing nothing as it gets overwritten when saving the dataset).

Instead of keeping a copy, we could make a read-only property out of it and even point it to the actual DatasetMeta object.

We can also consider DRYing dataset metadata that is saved in both index.json file and [table].meta.json files (and load dataset's metadata when loading the table).

[larsyencken]

This PR adds the unit test for this situation: #1276

[larsyencken]

Calling this one done after the merged changes.

[Marigold]

This has been fixed in ETL steps, but it's still a problem in core ETL. It has caused some nasty bugs in the past. I'm not going to reopen this now, but I might if I run into this again in the future.

[larsyencken]

It will fail unit tests if you do this now, which in some sense is good enough to solve this specific issue.