Clean up hdf_archive (addresses issue #80)

  modified:   pytriqs/base/archive/hdf_archive.py
Rename data and tail accesses of GF

Changed calls like _data, _tail, gf_init which
are different with the new GF. Only the dft is
left unchanged for now.

  modified:   doc/user_manual/green/block.rst
  modified:   doc/user_manual/wien2k/advanced.rst
  modified:   pytriqs/base/dos/hilbert_transform.py
  modified:   pytriqs/base/gf/local/block_gf.py
  modified:   pytriqs/base/gf/local/gf_generic.py