add @mandresm contributions to the fix

[mandresm]

• Remove keep_list syntax (if we can avoid it, I'd rather do not add more syntax here)
• Makes sure it does the old list handling for date operations, so that date operations are still handled correctly
• Sets the provenance accordingly

This is tested and produces a list such as discussed in #1279

[mandresm]

Hey @pgierz ! This is a PR to your original branch with hopefully a few improvements. Feel free to merge if you like it :)

[pgierz]

Looks good, but I have a question. See below:

[pgierz]

Was this only the case for dates? I didn't know that...

[mandresm]

It was the case for every list, but I think the only way we had of generating lists from a string on the past was via Date operations (because date objects don't contain this functionality so that functionality ended up here, somehow)

[pgierz]

I might not understand something about provenance 100% yet, but why not this version:

Suggested change

	entry = ListWithProvenance(result, None)
	entry.set_provenance(rhs.provenance)
	entry = ListWithProvenance(result, rhs.provenance)

[mandresm]

For providing provenance values during instanciation of ListWithProvenance (or DictWithProvenance) the provenance needs to have the same structure as the original list or dict. That is to say, if you have a dict with some key-values, what you need to give in the second argument of the instancing is also a dict with the same keys, but the values are provenances. set_provenance is used to assign the same unique provenance to a whole list/or dict with provenance, which is what we want in this case (we are expanding a string into a list so we want to assign a single provenance information to the new elements of the list.