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.

I was going through the codebase, and I believe the implementation of __add__() and __iadd__() will be similar to concatenate_datasets() after the elimination of code for arguments other than the list of datasets (info, split, axis). 
(Assuming elimination of axis means concatenating over axis 1.) Most data frame libraries (Polars, Pandas, ...) override `__add__` to perform (mathematical) summation, so having different behavior here is a bad idea.