Bug Report: timestamp[ns] not recognized
The objective in terms of design is to make it easy to create Features in a pythonic way. So for example we use a string to define a Value type.
That's why when inferring the Features from an arrow schema we have to find the right string definitions for Value types. I guess we could also have a constructor `Value.from_arrow_type` to avoid recreating the arrow type, but this could create silent errors if the pyarrow type doesn't have a valid mapping with the string definition. The "round-trip" is used to enforce that the ground truth is the string definition, not the pyarrow type, and also as a sanity check.

Let me know if that makes sense 
Repro:

```
from datasets import Dataset
import pandas as pd
import pyarrow

df = pd.DataFrame(pd.date_range("2018-01-01", periods=3, freq="H"))
pyarrow.Table.from_pandas(df)
Dataset.from_pandas(df)
# Throws ValueError: Neither timestamp[ns] nor timestamp[ns]_ seems to be a pyarrow data type.
```

The factory function seems to be just "timestamp": https://arrow.apache.org/docs/python/generated/pyarrow.timestamp.html#pyarrow.timestamp

It seems like https://github.com/huggingface/datasets/blob/master/src/datasets/features.py#L36-L43 could have a little bit of additional structure for handling these cases?  I'd be happy to take a shot at opening a PR if I could receive some guidance on whether parsing something like `timestamp[ns]` and resolving it to timestamp('ns') is the goal of this method.

Alternatively, if I'm using this incorrectly (e.g. is the expectation that we always provide a schema when timestamps are involved?), that would be very helpful to know as well!

```
$ pip list  # only the relevant libraries/versions
datasets                      1.2.1
pandas                        1.0.3
pyarrow                       3.0.0
```
Bug Report: timestamp[ns] not recognized 
 Repro:

```
from datasets import Dataset
import pandas as pd
import pyarrow

df = pd.DataFrame(pd.date_range("2018-01-01", periods=3, freq="H"))
pyarrow.Table.from_pandas(df)
Dataset.from_pandas(df)
# Throws ValueError: Neither timestamp[ns] nor timestamp[ns]_ seems to be a pyarrow data type.
```

The factory function seems to be just "timestamp": https://arrow.apache.org/docs/python/generated/pyarrow.timestamp.html#pyarrow.timestamp

It seems like https://github.com/huggingface/datasets/blob/master/src/datasets/features.py#L36-L43 could have a little bit of additional structure for handling these cases?  I'd be happy to take a shot at opening a PR if I could receive some guidance on whether parsing something like `timestamp[ns]` and resolving it to timestamp('ns') is the goal of this method.

Alternatively, if I'm using this incorrectly (e.g. is the expectation that we always provide a schema when timestamps are involved?), that would be very helpful to know as well!

```
$ pip list  # only the relevant libraries/versions
datasets                      1.2.1
pandas                        1.0.3
pyarrow                       3.0.0
``` 
 The objective in terms of design is to make it easy to create Features in a pythonic way. So for example we use a string to define a Value type.
That's why when inferring the Features from an arrow schema we have to find the right string definitions for Value types. I guess we could also have a constructor `Value.from_arrow_type` to avoid recreating the arrow type, but this could create silent errors if the pyarrow type doesn't have a valid mapping with the string definition. The "round-trip" is used to enforce that the ground truth is the string definition, not the pyarrow type, and also as a sanity check.

Let me know if that makes sense 