Make to_dataset method work with copy

[alex-hse-repository]

🚀 Feature Request

Make TSDataset method to_dataset work with copy of the passed dataframe

Motivation

Some users argues that it is not obvious that to_dataset method modify the dataframe inside

Proposal

Now to_dataset method should work with copy of the dataframe

Test cases

• In tests/test_dataset/test_dataset.py create test test_to_dataset_not_modify_dataframe
• Use some fixture as an input of the to_dataset method and make sure it does not modify the passed dataframe

Alternatives

No response

Additional context

No response

Checklist

• I discussed this issue with ETNA Team