Make `TSDataset` slice return `TSDataset`

[alex-hse-repository]

🚀 Feature Request

Now the slice form TSDataset should also be TSDataset

Proposal

1. Change current behaviour of __getitem__, head, tail methods -- now it should return the corresponding TSDataset
2. Method __getitem__ should work using the following rules:

• If 1 item -- [timestamp]
• If 2 items -- [timestamp, feature]
• If 3 items -- [timestamp, segment, feature]
• timestamp = int, str, slice[int], slice[str]
• segment/feature = str, list[str] or ":"

4. Remove methods loc, tsdataset_idx_slice
5. Fix all the places, where the slice of dataset is used including notebooks
6. Improve the existing tests

Test cases

1. Method head(10) works the same as ts[-10:]
2. Method tail(10) works the same as ts[:10]
3. Test all the possible cases of __getitem__

Additional context

No response