Support various methods for Attributes

[simetenn]

• [ ] __delitem__()
• [ ] Flag for storing all attribute information, atm all information on for example numpy objects are lost

Properties in h5py that we could support:

• [ ] clear
• [ ] create
• [ ] get
• [x] items
• [x] keys
• [ ] modify
• [ ] pop
• [ ] popitem
• [ ] setdefault
• [ ] update
• [x] values