Make construction of AttributeDict recursive

[sphuber]

Fixes #2996

This means that dictionaries nested in the dictionaries will also
recursively be turned into AttributeDicts. This has the benefit that
when constructing a nested dictionary like:

dictionary = AttributeDict({'sub': {'name': 1}})

the nested keys can all be accessed as attributes, i.e.:

dictionary.sub.name

whereas before one had to do:

dictionary.sub['name']

which is not very intuitive.

[coveralls]

Coverage increased (+3.06%) to 72.957% when pulling f6bed23 on sphuber:fix_2996_make_attribute_dict_recursive into 6c544bf on aiidateam:develop.