Issue #1895: Bugfix for string_to_arrow timestamp[ns] support
OK! Thank you for the review - I will follow up with a separate PR for the comments here (https://github.com/huggingface/datasets/pull/1900#discussion_r578319725)!
Should resolve https://github.com/huggingface/datasets/issues/1895

The main part of this PR adds additional parsing in `string_to_arrow` to convert the timestamp dtypes that result from `str(pa_type)` back into the pa.DataType TimestampType.

While adding unit-testing, I noticed that support for the double/float types also don't invert correctly, so I added them, which I believe would hypothetically make this section of `Value` redundant:

```
    def __post_init__(self):
        if self.dtype == "double":  # fix inferred type
            self.dtype = "float64"
        if self.dtype == "float":  # fix inferred type
            self.dtype = "float32"
```

However, since I think Value.dtype is part of the public interface, removing that would result in a backward-incompatible change, so I didn't muck with that.

The rest of the PR consists of docstrings that I added while developing locally so I could keep track of which functions were supposed to be inverses of each other, and thought I'd include them initially in case you want to keep them around, but I'm happy to delete or remove any of them at your request!