Data needs to be more precise

[egull]

Olivier:

• we can not make arrays of complex in hdf5. We store them as an array of dim +1
  where the last index (fastest hence) is 1,2, for real and imag. part.
  Besides, we tag it with the attribute "complex".
  This follows ALPS2 convention.

[egull]

Olivier, we've changed the specification. Please comment and close if approved.