Rewrite and attempt to make more concise the 'data' and 'data-lifecycle' documentation.
Remove obsolete weakness (now corrected) from mapping.rst
Document the database structure using sphinx autodoc and a bunch of docstrings.
Use headers less sparingly, for more useful linkable sections.
Merge branch 'doc-improvements'

* doc-improvements:
  Use headers less sparingly, for more useful linkable sections.
  Document the database structure using sphinx autodoc and a bunch of docstrings.
  Remove obsolete weakness (now corrected) from mapping.rst
  Rewrite and attempt to make more concise the 'data' and 'data-lifecycle' documentation.
Merge branch 'master' into links
Added map_dict property for Item.
Extracting value from map of linked item.