concatenate_datasets support axis=0 or 1 ？
For the API, I like `concatenate_datasets` with `axis` personally :)
From a list of `Dataset` objects, it would concatenate them to a new `Dataset` object backed by a `ConcatenationTable`, that is the concatenation of the tables of each input dataset. The concatenation is either on axis=0 (append rows) or on axis=1 (append columns).

Regarding what we need to implement:
The axis=0 is already supported and is the current behavior of `concatenate_datasets`.
Also `add_item` is not needed to implement axis=1 (though it's an awesome addition to this library).

To implement axis=1, we either need `add_column` or a `ConcatenationTable` constructor to concatenate tables horizontally.
I have a preference for using a `ConcatenationTable` constructor because this way we can end up with a `ConcatenationTable` with only 1 additional block per table, while `add_column` would add 1 block per new column.

Maybe we can simply have an equivalent of `ConcatenationTable.from_tables` but for axis=1 ?
`axis` could also be an argument of `ConcatenationTable.from_tables`
I want to achieve the following result
![image](https://user-images.githubusercontent.com/12437751/99207426-f0c8db80-27f8-11eb-820a-4d9f7287b742.png)

concatenate_datasets support axis=0 or 1 ？ 
 I want to achieve the following result
![image](https://user-images.githubusercontent.com/12437751/99207426-f0c8db80-27f8-11eb-820a-4d9f7287b742.png)
 
 For the API, I like `concatenate_datasets` with `axis` personally :)
From a list of `Dataset` objects, it would concatenate them to a new `Dataset` object backed by a `ConcatenationTable`, that is the concatenation of the tables of each input dataset. The concatenation is either on axis=0 (append rows) or on axis=1 (append columns).

Regarding what we need to implement:
The axis=0 is already supported and is the current behavior of `concatenate_datasets`.
Also `add_item` is not needed to implement axis=1 (though it's an awesome addition to this library).

To implement axis=1, we either need `add_column` or a `ConcatenationTable` constructor to concatenate tables horizontally.
I have a preference for using a `ConcatenationTable` constructor because this way we can end up with a `ConcatenationTable` with only 1 additional block per table, while `add_column` would add 1 block per new column.

Maybe we can simply have an equivalent of `ConcatenationTable.from_tables` but for axis=1 ?
`axis` could also be an argument of `ConcatenationTable.from_tables`