lib.config: allow attributes to be recursively applied to struct items

[Morg42]

closes #723

You can insert item structs into items:

item1:
    struct: foo

or

item1:
    struct:
    - foo

If you want to recursively add attributes (e.g. enforce_updates: true or database: init or visu_acl: ro...) to all items imported from a struct, you would have to explicitly and manually define every item.

With this PR, you can do this as follows:

item1:
    struct:
    - foo:
       - enforce_updates: true

Note that:

1. even for one struct, you need to notate a list with dashes,
2. after the struct name, you have to add a colon : to make this a dict in yaml. Omitting the colon will prevent shng from being able to parse the yaml file.

You can add multiple struct (in the usual list format), and you can add multiple attributes (also in list format as shown). And of course you can add different attributes each time you include the same dict...

i_fn:
    struct: 
    -   my.attr.sf:
        - value: 'abc'
    -   my.attr.sn:
        - value: 2

i_f:
    struct:
    -   my.attr.sf:
        - value: 'xyz'

i_n:
    struct:
    -   my.attr.sn:
        - value: '1'

Adding nested dict structs is possible, but might lead to unforeseen consequences, as nested attributes introduce subitems to all items...

i_n:
    struct:
    -   my.attr.sn:
        - subitem:
               value: '1'

would add item subitem with value 1 (and implicit type foo) to every item in the struct...