[h5dataset] change string conversion to work with more than 1d

[jgrewe]

fixes a problem with 2d string data

[coveralls]

Coverage increased (+0.03%) to 79.661% when pulling e583bcb012a5aa1738c426f99af3e85e67eb47c6 on jgrewe:str2ddataarray into b139decb5111f86ce8953c9fc0ee2cbc96733498 on G-Node:master.

[jgrewe]

@mpsonntag you are perfectly right and I admit, I was simply too lazy to do it even though I thought about it...