Read in Excel Rows or Columns as 1dimensional lists and arrays.

When reading in horizontal/vertical Excel Ranges, xlwings currently returns a nested ("2d") list:

```
[[1,2,3,4]]
```

They should be returned as flat list using something like:

```
[item for sublist in l for item in sublist]
```

in order to get:

```
[1,2,3,4]
```

Same actually for vertical lists `[[1],[2]]` and for numpy arrays.

Add argument `atleast_2d` to force 2d behaviour.
