Add `__add__()`, `__iadd__()` and similar to `Dataset` class
**Is your feature request related to a problem? Please describe.**
No.

**Describe the solution you'd like**
I would like to be able to concatenate datasets as follows:
```python
>>> dataset["train"] += dataset["validation"]
```

... instead of using `concatenate_datasets()`:
```python
>>> raw_datasets["train"] = concatenate_datasets([raw_datasets["train"], raw_datasets["validation"]])
>>> del raw_datasets["validation"]
```

**Describe alternatives you've considered**
Well, I have considered `concatenate_datasets()` 😀

**Additional context**
N.a.
