Unify read/write routines for Datasets and Attributes

[ferdonline]

Datasets and Attributes duplicate much of the code to read and write data. In #277 initial work has been done separating read/writing logic into its own source file. If would be desirable to make such routines to work for Attributes as well, with eventual minor changes.

[alkino]

You would like that Attribute inherit from Slice_traits?