8201 Fix DataFrame subsets indexing in CSVDataset()

[bartosz-grabowski]

Fixes #8201 .

Description

convert_tables_to_dicts was using .loc to index DataFrames which was changed to .iloc. It was causing unexpected behavior in CSVDataset as demonstrated in #8201 because .loc expects labels, but was instead provided positions of the rows.
Unittest was added which fails before the change and passes afterwards.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[bartosz-grabowski]

/black

[bartosz-grabowski]

@KumoLiu @ericspod could you take a look at this?

[ericspod]

I think this is good to go, we're waiting on a large PR to be merged, then we'll update this one and move forward.

[KumoLiu]

/build