String size cannot be set for individual datasets

[Apollo3zehn]

It should be implemented like OpaqueInfo because not only the string size is relevant but also the char set and string termination type. There might be some applications that require these specific details. It should not be added to H5DatasetCreation because it is also relevant for attributes. Maybe implement it via new type H5StringDataset(xxx, xxx). I don't know what is the better solution. But there might be a reason why OpaqueInfo was created instead of H5OpaqueDataset.