Variable or fixed length string data

I've been looking into supporting string type data in DataArrays (see old issue #82).
I think this is doable but there's one decision to consider and I would like some feedback.

We can support variable length or fixed length strings as data in DataArrays, or maybe both in some special way, and here are the arguments I can think of for each.
## Variable length
- Obviously this is more flexible. Once a DataArray is created it can be edited freely.
- We already use variable length strings in [Property](https://github.com/G-Node/nixpy/blob/2d33d77fe9ccca2ea7065918c32e096026e2444f/nixio/pycore/property.py#L114) for string type metadata and [Dimension](https://github.com/G-Node/nixpy/blob/86ff40667a423c3640697be4cabe929dbd88d3b2/nixio/pycore/dimensions.py#L269) for labels.
- **Potential issue**: The [special dtype](https://github.com/G-Node/nixpy/blob/10e48a3b83bffed7e68d7a5fb5352b4d8e69d74a/nixio/pycore/util/util.py#L34) we create to support variable length strings is simply of type `object` (`np.dtype('O')`). This isn't an issue now since no other data type we use matches this type. We could just assume that any data we read back that's of type `O` is a variable length string.

## Fixed length
- We could restrict strings in DataArray to be fixed length, the same way any other type of DataArray can be reshaped but not resized. A string in this instance would be useful for storing vectors of character data and not necessarily text.
- The stored data type would not be lost on read. We would get back a `np.dtype('|Sn')`, where `n` is the length of the string.

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I'd love to hear everyone's thoughts on this.